Sion gets way too much praise for being healthy when he isn't
and, funny enough, all the praise for Sion being an amazing champion only ever seems to come out when he's not good, and you're really hush whenever he's stomping out all the competition in top lane.
For all the bashing my champions (Juggernauts) seem to get for being "either too good or too weak" or "not interactive" Sion seems to take all of these traits much further. Any time that Sion is good, he is an insanely oppressive force - be it from top lane, mid lane, or even the odd support pick. Not like "oh he's gonna snowball off me when I play sloppy because this is Silver-to-Plat ELO," like Darius commonly gets attributed to; Sion goes all the way up the top of the rank and pushes a ton of champs out of his lane. He's not really that interactive; most times I get to actually play the lane against a Sion is when they're a bad Sion who wastes their spells. Against a good one who lines up a Q any time you go to farm, or pre-E nerf smacking you with his E max spam any time he could, I didn't feel like there was a whole lot of interaction; I can either not farm, or I can eat shit. And unlike Darius, Sion's poke/all-in is much lower cooldown. Darius has 24 seconds from levels 1 to 7 on his E; plenty of time that you can hit back, and if he missed that E or it got blocked, it's practically a death sentence in most non-tank matchups.
Sion's Q starts at a 10 second cooldown and goes down 1 second each rank; with the current Q max shit, that means by level 7 his Q will be on a 7 second cooldown. Add in the fact that it will do a lot of damage even with less than half a charge and you can see why he's hard to deal with in top lane. Adding on to this is that CCing him to cancel his windup does not really reward you, since this refunds his Q cooldown to 2 seconds. You will almost never be able to disengage from trading with Sion before his Q is back up off CD, especially if you aren't mobile. You can play the lane correctly, cancel his Q, dodge his E, but because of this mechanic Sion can catch you walking away and immediately nullify your advantage - or put you on the defensive for even trying to fight with him. It heavily discourages interaction from the majority of top laners - a design that, by your standards, is a problem.
Sion outranges most fighters between Q and E, he's more valuable to a team since he's a tank with long-range initiate and a shit load of CC, and he's fairly good at answering fighter splitpushes thanks to Q's waveclear and his own ability to split competently and group with ult, int strat notwithstanding. His main weakness is, funny enough, very similar to Juggernauts (the champ group most of you despise with a passion for completely arbitrary, usually rank-related, reasons) - being telegraphed, which lets ultra-mobile champions laugh at him after they've got enough CDR to dance around (but he usually obliterates them before then anyway), and being immobile, which has a reduced impact on him compared to real Juggernauts thanks to Q CD reduction, his unstoppable ult and the fact he doesn't "qualify" as a damage threat on the level of Darius or Illaoi - given that he plays as a tank who just happens to have obscene damage, he's perfectly fine with soaking up CC and his R lets him ignore it too.
I'm already aware that the double standard exists entirely because of "muh skillshot" syndrome (even though Illaoi is just as if not more skillshot dependent but you hate her too), so I don't really feel the need to ask. But I guess I'd like to end the post by saying that skillshot =/= interactivity and I'd much, much rather go back to playing against ridiculously snowbally matchups in top lane like Riven or Nasus than I would want to play a single match against a Sion who's any tier above 'dog shit.'
TL;DR fuck Sion, stop praising his shitty design, thanks