I'm just curious, why are there not similar complaints about other microless situations that other units cause? Forget balance, winrates, etc. I'm referring to the specific arguement that fungal is poorly designed because all you can do against it is set up prior to the engagement. The same arguement could be made against colossus since once you get enough of them, no amount of micro can really save your army if you attack with everything in a big clump, not because of any movement impairing effect, but because everything just evaporates so quickly. The same coukd be said of ghosts against Protoss with emp, toss can't really do any splitting while emps are going off, they either pre split or they lose almost all energy and shields on everything they have. There are plenty of other scenarios where how you position/prepare for an engagment will basically dictate how the battle goes regardless of any micro done during the battle itself.
Setting up before is one thing. As in your example, you have to set up properly to engage, right? So what is the difference, well it's simple:
You can leave. You can kite. You can try to get into a better position. You can blink. You can siege. You can burrow. You can do almost anything else once you get in there.
infestors negate everything. You can't run away if you get into one bad engagement. If they blind-side you, you're a dead man. They're basically sentries with cloaking whose FF's do added damage and prevent all abilities as well as demolish air units. Not to mention they basically start with a slightly different version of Halluc (which sentries should start with! good job on that blizz) than can do damage.
Sure, against huge numbers of other casters, pre-splitting is highly advised, no one is saying it isn't. however, getting carpeted in EMPs doesn't mean you have zero chance of escape like it does with infestors. Hell, I'd figure infestors are much scarier in death-balls than ghosts will ever be. The only real way to deal with broodlords is to blink or archon toilet. Fungal stops both, not to mention infestors also counter the mothership.
The only ability in the game that has the same level of conditions on it is vortex. Vortex can only come out of a t4 unit that costs 400/400 and is slow as molasses and costs 100 energy. Fungal comes off a t2 unit that's mobile, cloaks, and has 2 other useful spells and costs 75 energy.
So why not compare the Infestor to the Tank? They both do a pretty good job of keeping the enemy from blindly charging into an area, due to having long range aoe.
I would love if 4 tanks could realistically kill entire armies if my opponents made small mistakes. That would be awesome. Sadly, that doesn't happen. Think about the last time you ran mutas into thors and the last time you ran mutas into infestors. Which one did more damage? I'm willing to bet the infestors did.
When was the last time you had a marine army run into a siege line? How many marines did you lose? I've run that a bunch of times, you run into a siege line and you can back out losing about 10 marines tops. You dart into an infestor line, you lose every marine, all your medivacs, and possibly all your tanks since you now no longer have support for your tanks.