CS has a concept of "trust factor" which groups people more likely to be cheating in the same game. If you ever queue with a friend with a low trust factor (which happens a lot if they're on a smurf account), there are cheaters in pretty much every game... seeing that many blatant cheaters has really made me think there must be a lot of smarter cheaters flying under the radar in regular games. It's basically impossible to tell whether someone is wallhacking or if they have really good game sense.
I've played for many years including CS1.6, CS:S, GO, and 2. I regularly snoop profiles and overwhelmingly there are accounts with 1 game, all F2P games, short account lives, low hours (CS2, GO + account), low or no commendations, low match count, blocked stat tracking... Slews of markers that these are less-than-legit, either smurfing (IMO a form of cheating) or outright hacking. Then they coincidentally overperform for their respective rank. That's without mentioning coming across numerous individuals with [sometimes multiple] bans recorded on their accounts.
My account is 20 years old. And has several games with hundreds of hours including a couple of perfect games, not to mention a hundred or more games. Also phone verified. I don't expect everyone to have similar accounts, but it's seldom I'm matched against anything even remotely similar, say 5 year old accounts with similar playtimes in non-F2P games, though many profiles are private, which itself is - I think - also suspicious since virtually everyone leverages aliases on Steam so I can't really imagine a case for this other than obscuration, though I'm certain some people do it for privacy reasons I expect that rationale is rare.
Beyond that I would say there are a lot of suspicious individuals I've been matched against in both premier and comp . Regardless of whether or not they're smurfs it makes MM obnoxious if only because you end up matched against people who rage and ruin 45m-1h of your time by competing illicitly.
The MM algo is also just shit without these considerations lumped on top of it. I regularly play with my friends who rank lower and that draws my rank down so we get matched in low ranks, resulting in violent pubstomping. Of course I play on my only account, so I'm sure I get hackusated a lot, which would ostensibly get my trust factor drug through the mud. I suppose that's a solid incentive for smurfing on its own, especially since the system is opaque.
It's all pretty bad, frankly. Faceit is hardly better, a lot of the community is pretty toxic and obnoxious salty tryhard metabangers that aren't fun to play with.
