D20 Future.
I was so excited about the product before it came out. I even prepared to run a campaign, until I noticed the large number of gaping holes in the rules. For many chapters, there was a different reason you couldn't use it as written.
The ship rules were so poor (parts of combat, but especially ship construction) that I would create a new set, and design a ship for the players. They insisted on designing the ship themselves, which meant I would have to make the massive set of ship construction house rules readable to people other than myself. That ended that campaign idea.
Another person in the group actually ran a campaign. He put very few restrictions on what we could be or do. So, for instance, we ended up with a ship and a mecha. Considering the speed difference, the mecha was worthless; the ship could land faster in a planetary atmosphere than the mecha. Why did we have mecha and combat spaceships in the same setting? That's probably more a campaign problem than a rules problem.
D20 Modern has broken Wealth rules, but since money is basically useless it wasn't a problem. (In a campaign I ran, the players got wealthy enough to buy a Swiss castle, but it wasn't hidden, the cops and crooks knew where they were, it didn't give any political advantage (they weren't nobles), it wasn't really more secure than a security-conscious house, etc.) Unfortunately, D20 Future made money useful (very useful) without fixing the ability to obtain sky-high Wealth. So we had one player buying a mecha, one player buying a ridiculously powerful robot with eight limbs (each of which was capable of attacking) and one player bought a secondary spaceship which, due to the broken spaceship design rules, was actually more powerful than the base ship. (This person played the main ship owner, which meant our commander wasn't even on our ship in combat).
The same spaceship buyer also "bought" a powerful robot, mainly a healer who had stats 24 around the board (there's no balancing robots). She wasn't deliberately designed to be a combat robot, but was as powerful as an NPC in her own right. (It was basically DnD 3.x's Leadership, but without a cost that can be balanced, and could exceed the PC's own power.) Note that this wasn't the eight-legged robot. Speaking of which, the eight-legged robot had a smaller robot "piloting" it. The small robot had high Dex, and would control the big robot's gun (and so get a much higher attack bonus than the small robot). The owner wasn't that great at combat (lots of Smart levels) which is why they did that.
We found the gadget system to be broken, mainly due to Modern's Wealth rules. Also, most of the non-combat gadgets were useless while most of the combat gadgets could do only two things: boost Defense, or boost damage by a lot. Considering the baseline wasn't changed (eg no more hit points, no higher massive damage threshold), combat got messed up. You couldn't hit often, but you could have a 3d12 rifle that, if it hit, would pretty much guarantee a forced massive damage save.
I forgot to bring the spaceship combat rules "supplement" (a post on this forum about how to make Pilot do something in combat) so the ship combat ended up being pretty boring, at least until the boarding action. (That's hard to screw up, I would think.)
