yourbeliefs summed up my thoughts on Gears Ultimate pretty well. I did have fun going back to one of my favorite 360 games but there were quite a few bugs and inconsistencies too.
One example that immediately came to mind is the kryll section where you are immediately eaten by piranha bats if you step into the dark. Part of the way through this you have to shoot propane tanks to provide light in sections you couldn't otherwise get through. For whatever reason we died after I had already shot one of these propane tanks bringing us back to our last checkpoint. After we got past all the enemies and back to this propane tank I remember telling
yourbeliefs "oh that's weird, this propane tank is still lit from last time we died." I jumped out the window and was immediately eaten by the kryll even though it showed that I was in the light on my screen. Yourbeliefs told me that on his he was just seeing a normal propane tank that wasn't lit yet.
This is just one example of us seeing different things displayed on our TVs even though we were playing together in co-op. It happened a few other times throughout the game.
There were other fun bugs too. At one point snapping to cover wasn't working for yourbeliefs but everything seemed normal to me. He paused the game and when he unpaused the issue was resolved for him but suddenly I couldn't get into cover and I immediately died.
The theater section they brought over from the PC version was incredibly frustrating. Yourbeliefs already mentioned this but we spent about an hour dying over and over again at the same checkpoint here before finally giving up for the night. I've never been one to throw and break my $60 controllers out of anger from something in a video game but that night was probably the closest I've ever been. As I predicted right before we quit, we beat it on the very first try when we came back to it a couple days later.
One of the reasons we kept dying is that you kill a seeder in this room and when you do a mini cutscene of him dying displays. Whenever this happened my character would stand up out of cover on his own with 2 guys on turrets shooting at me and 3 wretches that spawn about 2 feet behind my character. Since we were playing on hardcore it only took about one hit from the wretches combined with the turret fire to kill me. Even if we took out the turrets before the seeder the wretches could down me in two hits and they spawned so close that it was difficult to deal with them sometimes.
This brings me to my next point. If the game didn't feel the need to constantly keep splitting us up when we were playing together in co-op the frustration factor from these sections of the game would have been gone. Normally your partner can revive you unless you are killed by something that completely destroys your body. You only have to go back to the previous checkpoint if you are both down at the same time. This did happen from time to time but it at least felt fair. In the frustrating theater section yourbeliefs was sniping from a balcony and I was on the floor below so he couldn't do much to help except for provide some cover fire.
At some points in the game it made sense for us to split up but in other cases it felt arbitrary. In an underground section it makes us choose which side of the glowing river each of us will be on. There is absolutely nothing that needs to be accomplished on either side of the river. It's just there to split us up so we can't revive each other. We can see each other and help out by shooting enemies on the other side but we can't revive until we make it to the end of the river.
It got to the point where I immediately knew it was going to be a pain in the ass every time the game asked us to split up. This obviously would have been easier if we played on normal difficulty instead of hardcore but then all of the parts where we are together would have been a cakewalk.
Finally, as yourbeliefs already said, the big boss fight at the end glitched out for us. I remember dying over and over again at this part when I played the original game back on 360. I think we only died twice here because RAAM got caught on the geometry and couldn't rush us. Then it was just a matter of killing the bullet sponge boss who couldn't fight back. It kind of took the excitement from beating the big bad boss out of the game.
I know it sounds like I'm being overly negative but I was always a Gears fan over Halo. It was THE reason to own a 360 for me. The weapons are still super satisfying to use and the gameplay is still fun for the most part. It was just a little disapointing to see how poorly one of my favorite games has aged in some areas. I had a really good time playing through it again 9 years later except for a few extremely frustrating sections. I think a large part of the reason I liked it is because I was playing through it in co-op for the first time. I'm not sure I would have enjoyed it much on my own.
Edit: As yourbeliefs said, we didn't get the achievements for beating every mission in co-op, beating every mission on any difficulty, beating every mission on normal, and beating every mission on hardcore that should have popped when we finished the game even though we played every bit of it together.
I have every achievement that pops when you beat each individual chapter but none of the ones you get for beating all chapters. How does that make sense?