AWKWARD ZOMBIE

With so many players online, the majority of them aren't very good. (either by lack of skill or still learning)
Spy is a difficult class to play, so you don't see a lot of 'good spies' using the spycicle. Therefore, Valve doen't get complaints about it. I could say something similar for heavy.
Pyros on the other hand are easier to understand, but likewise present the players that don't know how to deal with them. Hense the complaint floods and excessive nerfing.
It's all player psychology and business. More cheddar comes from the mass of players so they're the ones to keep pop flyin'.

the spycicle needs some kind of downside but it's hardly OP, five seconds of fire invincibility isn't that great when the pyro can just take out any other non-fire weapon and shoot you

and then it lets everyone know there's a spy around because the backstab noise is distinct as hell

and heavy is the opposite of OP seeing as every class has a counter against them

Or just keep chasing you with fire like a dippy-doo.

The big ice statues are a dead give away that a spy is around. Hitting a cloaked or disguised spy with the Spicicle gives off a sounds that the knife melted so it's not as useful unless you're around a lot of gunfire and other noise.
Also you can't backstab heavies with the Hue Long Heater, which I see a lot of actually.

spycicle is basically for getting away from pyros, pop your cloak and/or dead ringer and if you are a smart juker you can generally get out with your hide intact. this is still counter by smarter pyros who will try to airblast you into a corner, cause when you are actually airblasted it gives a distinct sound opposed to airblasting air itself.

Generally I see pyros (including myself) just making informed guesses as to where the spy is going and just not stop flaming. 2 seconds is not a whole lot of time to get away, especially on smaller maps.

False, I've backstabbed many a Hue Long Heater Heavy before my knife melted.

I've also airstabbed one during a game of Hightower (Dude was on the elevator before the finish, I jumped down from the little platform and killed him without getting set on fire)

there's an interval between the flames which, if you time it correctly, allow you to get a stab without being set on fire

also i find that a dead giveaway that a spy is around is killfeed notifications coupled with teammates screaming

Once a Spy backstabs, everyone knows he's around. The first kill is always the easiest. After that, you gotta catch em' on guard.

If you stab them at the height of your jump, the particles will disappear before you touch them. Heavy's stabbox is just barely tall enough for that to work.

Don't forget about Snipers.

Nobody complains about snipers because the only people that WOULD complain about snipers ARE snipers.

snipers have the amazing skill of being hated no matter what they do. are they bad? they're wasting a slot. are they good? they're aimbotting pieces of shit that kill fun. are they decent? they don't get kills when needed and then they get bodyshots.

also spies are a great class for getting a pick kill and then switching because the enemy team will be on the edge for a while until they see that the spy swapped but 'changing classes' is a concept that is too foreign to most players for a game where classes are a central concept

I don't hate snipers. I find good snipers challenging and bad snipers fun taunt kills.
God forbid I have fun while playing a video game.

The spycicle doesn't give killfeed notifications or screams. It's a practically silent killer, but on the flipside the statues they drop VISUALLY are a dead giveaway. So it's not great for camping in one area but it is good for being a good spy, moving around and getting chainstabs relatively unnoticed.

Plus the fire block gives you some time to revolver that pyro dead if you're using ambassador or maybe diamondback. The cloak-juking strategy isn't bad though, but as said before they can airblast you or shotgun flicker you.

All in all I prefer it to the regular knife and use it as a solid substitute when I'm not doing my fez + eternal reward + l'etranger sandstorm taunt shenanigans.