I've enjoyed these games to a point, I own a couple of the versions, mainly the first one, and the last one (UNIVERSAL COMBAT COLLECTORS EDITION YO.) Anyways...
First, the pros:
The gameplay is something I've always wanted, a game where you can fight in space, in air, or on land. Be a marine, be a pilot, etc. It's a awesome idea in perspectives but it has a HUGE learning curve, and if you don't have an IQ of at least 120 (sorry Americans

), hell if you'll ever get the gameplay down.
The game basicly allows you to fly a spaceship, be able to take a shuttle down to a planet (or take your spaceship, but it takes hellish long.) land, unload a vehicle such as a tank/jeep/etc, or drop a mining vehicle, etc. Or just simply get in a space fighter or air fighter and duke it out. Being a marine only is simply horrible because you cannot leave the planet due to not having a space fighter skill to be able to leave orbit. The only worthwhile status to be would be a commander. But enough of that.
One neat thing is it actually has OTS type missiles too, which are orbit to surface. Some of them can even destroy everything on a planet, which seems silly, highly expensive, if you even get there before you're pilot dies.
Another great thing is the fleet you can control as you gain ranks (I'll discuss ranking below, you know, in the cons.) But virtually you can have a fleet to help you out as you gain ranks. (You always start off as the lowest rank, normally.) But controlling a fleet is great, because you can start capturing starbases. Two ways to capture a base, critical the starbase and take it over, or cut off its supplies (Which takes quite awhile but can be easier if your just starting a fleet.)
Damage is pretty realistic, you can loose out on pieces, of the bridge, etc etc. You can also eject a few things, and if your AI is smart enough during a radiation leak, they will eject the reactor, and you'll need to get towed. Otherwise you'll probably be destroyed and or have some radiation through out the ship, where you'll need your doctors to give out there med packs/radationpacks.
Everything is real time, even repairing, which can take hours if you are not docked. The game is ment to be played over long hours, it's not made for the person who sits and expects everything to come at them in 5 minutes. Sometimes you can expect to wait 30 minutes for a enemy ship to arrive in a mission. I kind of enjoy that, but that is just me. The whole 'real time' feel is pretty fun.
The fun thing about it is you need to maintain and upkeep your command ship (Given your playing commander), food, supplies to power the ships reactor, shields, weapons. You can also upgrade the reactor/shields/etc, cept not weapons. Weapons is defiantly on the con list.
You can gather materials normally by mining, but there are some minor different ways too. Overall its pretty neat how you can control everything, such as your fleet, vehicles in your bay, etc. You can also transport/teleport to a surface, and might I add, you can transport ALOT at a time. It's pretty interesting since you have about 200 marines sitting in your command center

.
The cons:
The learning curve is horrible. The instruction manual is 96 pages, and the tutorial is another 100 pages. Seriously, it's just that big in sense. But honestly, they made it too complicating because of the command ship upkeep and what you have to do with it. Otherwise the game is just another shoot shit in space mostly. Which depresses me.
Capturing starbases is quite fun, probably the only real fun thing to do in space. There is ultimately no point to land on a planet with marines and vehicles to kill enemy bases because you cannot capture anything! And they all rebuild after about 30 minutes. Only for a little bit of rank.. Ultimately, that does get boring. I asked Derek Smart (The creator about it.) And he basically said "Meh, I won't be creating another UC probably, so won't be adding that." Basically, it was supposed to be added, but never was, thus it makes marine and planet stuff BORING after a short while.
Ranking takes so long to get, and if you die, your dead, you'll hope to of saved something, but normally you don't and your profile is now 'useless' That's a huge annoyance. And when you save, you are exited from the game to the main screen, which is a fucking pissoff because the game takes ages to reload. Basically, you never want to save, and if you die, your dead. As for ranking, the game only really starts when you gain some and your crew gain AI, because your all 'retards' with about 20IQ.
Commanding a shuttle to land on a planet and unload marines/etc is a bitch as well, (Yes I did read the HOWTO on it.) Reason being, your AI either crashes into the planet normally and takes huge shuttle damages, or it slowly.. slowly.... goes down, which if you leave the game up for several hours, they've gone down a couple inches. This ultimately gives me a headache. Otherwise the AI are pretty smart other then that. I normally end up going through all my shuttles and unloading the mine drones myself, which is a bitch. (Need my money though.) Probably the thing that would make the game 1000% better would be THIS.
Online play is totally hacked. If you play online, theres no save profile feature, so when you log out, you lose everything. It's useless for a game that is ment to be played weeks/months etc. It's basically like levelling a character in a MMPOG, then having it wiped because you relog. Gaining rank is SLOW, again. So online is basicly USELESS other then hack and slash, and the 'online roam' feature is useless.
Expansions, there was only one worthwhile one that updated graphics to be a bit newer, but otherwise, they released 5-6 different expansions, that do the same fucking thing, but different scenarios missions (But normally people like to play free roam, scenario is really made for like marines since its that boring.) What the fuck is the point? Basically the last expansion was ALL The expansions put together, all the scenarios since he's ever created the game.. Heh. ONLY DIFFERENCE.
The weapons are too fast, they act like machine guns, so you can hardly dodge, and it makes difficult when everythings getting a super easy lock on you and you can only move as fast as your eyes do. AI are really overpowered in this one in a sense like how old games 'insane' mode were. Not to say, theres not many weapons, theres alot of missile types, but no 'primary weapons' which makes boring fighting. Might as well set it to AI and let your AI do the work.
Enemies on board is also annoying, when enemies are in the system, not even NEAR you, they can board you with hostiles, which is FUCKING annoying. At one point I had over 200+ in the cells,a for some reason I kept getting boarded, it seems to always happen, and it shouldn't when my shields are up, and that there not CLOSE to me. transporting is one thing, but getting boarded all the time when enemies are targeting you but are 999,999,999KM away, and your shields are up, is another. The other issue is, I go unload all the prisoners, undock, and tons more enemies board me again, just after i undocked, I ended up going back like 2-4 times since then there weren't enemies in the sector. This normally happens more in the game when your being targeted more often. It's really annoying.
Warping - The cheap way, basically when warping you can travel through planets, what a hack. The reason he gives is 'nothing during transisitions is real' or some junk like that, basically some garbage so that he doesn't have to code a work around for it. As well, warping is always the same duration, weither its 1000km, or 10000000km, its always 2 minutes, (real two minutes.) and there is NOTHING you can do about it. It's a stupid hack I feel that was added because he didn't want to add it proper. Seriously, what logic is that?
Last but not least, Derek Smart, the creator, he really doesn't like criticism about his game. I once made a post about how good the game was but the online capabilities should have a 'save' feature. He ended up locking it, and posting "No." I also see him hammering on others, like there a piece of shit for buying his game and giving advice to making the game better. That's something that really frustrated me, I never bothered going to his forums since, who knows what he is like now. It's basicly his game, and no one elses for input. That logic is bad after you know, more then 10 years of the same game being always the same with minor bug fixes, and just 'new scenarios'.
P.S. I still cannot believe he didn't add a way to capture planet bases, out of all things, he had a 'command center' was supposed to be for it, but yeah, whatever I guess.

/endrant.