After playing the game for a month, I think I am qualified to say this:
Illuminates are just plain shit to fight. Not hard, but tedious. It's a schizo faction that has no identity, except everything takes a million bullets to kill so lol fuck you if you wanted to have fun. No machineguns or explosive weapons to kill zombie hordes? Get fucked lol. Oh you wanted to strategically cripple the fleshmob so you can reposition? Lol, nothing slows it or even staggers it so have fun being ragdolled, dipshit. Also you gotta magdump it to kill it in the first place. See that overseer there? Prepare to be two-tapped by his staff, the first which ragdolls you and you won't recover in time before he delivers the coup de grace. Also somehow they can take more hits than the literal metalmen with rocket pods and shield. Oh you wanted to shield against that funny death beam of the walker? Sorry sir, you still get vapourised, can't let you counterplay that. How about the single most tanky enemy in the map that's supposed to be a map hazard, except its not and four of them can spawn and the only way to take them down if you bring the select few AT weapons and magdump it.
It really says for something that the stingray is probably the best designed enemy and it's just a gunship that strafes you from time to time. Harvesters are okay, aside from the instant death beam. But everything else? Man, nah. Not dealing with that.
My biggest beef with them is simple: The complete lack of counterplay options.
The illuminate have no mitigation or prevention conditions, and nothing that makes their units easier to deal with. The two biggest offenders are Fleshmobs and Leviathans.
FLESHMOBS
Fleshmobs have no weakspot - they're just 6000-HP sponges that have to be burnt down with raw damage. Antitank weapons don't work, direct impacting them with an Ultimatum doesn't work, and they can survive the majority of the game's airstrikes without breaking stride. They appear in bulk, absolutely everywhere, Ragdoll you on hit, and frequently clip into terrain where they're somehow able to keep hitting you.
The closest thing to a hard counter they have is a player with the Stalwart, which will kill them in about a third of a magazine, which at max ramp up will still take about four uninterrupted seconds on full-auto assuming every round hits. It will take about 6 and a half on the default 850 fire rate.
The only weapons that can one-shot them are Hellbombs.
At least 2 hits from the 380mm are needed to finish one off, and they can survive multiple 500kgs.
They appear on every difficulty and are a constant problem on many maps.
LEVIATHANS
Comprehensively broken. No way to solo one unless you specifically call in a very specific strategem (Anti Tank Turret or Exosuit), and it will use up most of the ammo for the latter (about a third of an Emancipator's autocannon ammo for both arms; the missile launcher on the Patriot will do it in about 11 (out of 14) shots if every single rocket hits dead-on), designed very much like they're supposed to be avoided rather than fought, but without any avoidance condition (like Bug Stalkers or Bot Gunships), no mitigation condition until relatively recently when you could remove the guns, and able to appear in bulk on some maps.
Running a Blitz and need to evac? Have fun with 2 of these obese trilobyte-looking motherfuckers taxiing over the evac point.
Oh, and they stunlock you and knock you down because of course they do. So if you run full Heavy Armor/Fortified for defense stacking, they'll pinball you endlessly. Wonderful.
WATCHERS
Not much to say here but for a tiny target, the Illuminate Bisexual Lighting Drone has 600 HP.
This is about 2 and a half times a Warrior bug on lower difficulties and nearly double a Warrior bug on harder ones.
It's also slightly less than a Berserker or Devastator. So if you're wondering why this tiny airmobile fuckface can somehow tank multiple direct impacts from plasma or being edged by a grenade launcher, this is why. It's also why you need an entire mag from the Warrant to put one down.
Because they fly, Watchers are basically impossible to escape once they've arrived; your only recourse is to kill them fast and hope that their shitty AI gets them hung up on terrain obstacles while you clear distance.
Oh, and their annoying little shithead shock slows you.
The entire Illuminate faction is designed to be as obnoxious as humanly possible. They were frustrating but not insufferable before the invasion; afterwards they became the single least faction to actually play against, and naturally that means the majority of MOs since have featured them.
Really makes you start to understand the Bugdivers.