Tremor is all about alien building and defending the alien base. Which a lot of players don't get alien building or defending the base. Tremor is basically 10 aliens outside the human base while 1 guy goes through the basement and kills their whole base. The alien default has 2 doors, which means you have a good opportunity for building and defense. I can beat aliens on Tremor simply because aliens get spread out and don't defend.
Arachnid is HORRIBLE for aliens. There are 0 doors on the whole map and it is wide open. Humans don't have to do anything spectacular, just don't camp. Most people think human building means 'find the best camping spot'. It doesn't. The default human base is spread out but easy to defend, as opposed to having a door to hide behind and spam out of. Arachnid is one of the least ground orientated maps and that benefits humans with jetpacks. A good human team should never lose Arachnid, regardless of alien defenders because it so big and open. Big and open=ranged wins.
ATCS is balanced. It can tip either way. The team that feeds least, wins. I think people are so used to get steamrolled 1 way or the other that they don't see it as balanced. ATCS is just so small that aliens get rushed by humans more frequently and they freak out. I can win ATCS on either team.
Karith is one of the better maps. It has open areas, tight areas, doors, long halls, and many base options. Its balanced to me. Its just that most humans don't go for the lift room. Most maps, aliens get the lift room first. But if humans get it, they can deny aliens the best base and force them into harder bases to defend.
ATCSZ Alpha is horrible to me, in general. Too easy for humans to camp in the default base. Unlike ATCS, this map doesn't have 2 entrances to the base. The vent doesn't count.
Ancient Remains, alien default is terrible. Its a good map but I don't really like how big and confusing it is. Maybe I could get used to it but smaller would be better if you ask me.
Flap, lol. Whoever builds up top, wins. Aliens can build away from their base better, so aliens have an advantage.
Gloom2Beta2, good map. Its all up the human builders and defenders. After that, humans just have to push out and have decent aim. The alien base is pretty much good to go, with minor tweaks. Its a rough map at first but I like now that I had time to play it a lot.
Nexus6, Niveus good maps. Good balance either way. Can't camp unless humans get the window room somehow.
The new Thermal is good. Smaller is better. Less ability to camp for any team.
UTCS swings to hard towards humans. Its okay but nothing really spectacular about it, other than the catwalks. Aliens don't have much options for a base. No doors on the map either.
Pulse, urg. Good map, just too painful to navigate.
Meep. Good map, too easy for humans to camp though behind portals or sky camp.
Uncreation, good map. Too easy for humans to get camped in by aliens but once humans get out of the base, the balance goes the other way. I guess that makes it balanced.
Transit, good map. Has lag issues. Egg spam is a pain but I rather egg spam than dealing real defenses. Most aliens don't know where to build on this map, but there are some nice places.
Hamunaptra, I hated this the map at first. Now, I consider it fun. Its big but its simple to navigate. It requires alien builders to be on the ball and so most alien teams lose.