The dlcs are a bit of a mess. The main thing that improved for me is that adding all this stuff allowed them to tone down the faction war, and factions no longer wipe each other out at a rate where you wondered why there were monsters in the game at all.
DLC 1 adds cybernetics.
The good:
It gives the player options.
It adds a new minor faction that for a while may attack havens, adding diversity to what you fight.
The bad:
The enemies are basically bullet sponge versions of New Jericho soldiers.
You are resource starved so may not have any resources to waste on something so trivial as cybernetics.
DLC 2 adds prehuman ruins and tech.
The good:
The weapons you research are powerful.
New enemies and map type are fresh.
The bad:
Once you have played one new map you have seen everything the dlc has to offer.
You have to manually collect the new resources with an aircraft hovering over the site. This is time consuming and costs resources if you have to craft an aircraft or two or three for this purpose.
At least in my playthrough I did not get resource sites for every type of ancient resources, making some tech uncraftable in my game.
DLC 3 adds fliers.
The good:
It adds a new layer to the game.
The bad:
Air combat sucks.
Aircraft are a huge resource sink.
The behemoth is the giant base they always wanted to add and with this DLC finally didn't add. It stomps havens but you can't interact with it. To drive it off you have to defeat enough fliers, which it randomly sends out. In other words you sit there passively waiting.
DLC 4 adds corruption and cloning.
The good:
Vat grown mutant clone troopers are cool and add variety to your roster.
Pandorans can corrupt havens, opening up cool though tough liberation missions.
Mutagen is finally useful outside Anu mutations.
The bad:
They overdid the need for mutagen.
Corruption is is a bad system as all it does is have you waste mutagen to cleanse your soldiers again and again and again. Corruption increases the damage your soldiers deal, but reduces their Willpower. Losing 1 willpower out of 12-20 WP your soldier may have per point of corruption is too big a problem compared to the 1% damage increase they get. It also means that soon enough the use of a single ability will get your soldiers close to panicking.
Clone troopers need mutagen to heal and to level up, but as you also need mutagen to cleanse your soldiers of corruption, it's not worth spending it on such a resource sink.
For every good idea they have, they come up with two to ruin the fun.