Server side only anti-cheat is one of the problem domains that I'd really love to work on at some point in my career. This is the type of adversarial arms race that just seems really fun to think long and hard about.
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It's so much easier to make cheats today than it was, say, 10 years ago.
It's also easier because more and more games are sharing common infrastructure like game engines, as compared to the past. What works in one Unreal game may save you a lot of time developing a cheat for another Unreal game.
These days, many online games encounter serious cheats within the first couple of days of release - if not the day OF release.
Cheating and anti-cheat used to rely a lot on the pure technical parts (like "is something sneaking some reads from the memory the game engine uses to clip models?"), which is ultimately not something you will win as a game developer (DMA/Hardware attacks or even just frame grabbing the eDP or LVDS signal and intercepting the USB HID traffic has been on the market for quite a while).
But implausible actions and results for a player can only be attributed to luck so many times. Do 30 360noscope flick headshots in a row on a brand new account and you can be pretty sure something is wrong.
If we can get plausibility vs. luck sorted out to a degree where the method of cheating no longer matters, that's when the tide turns. Works for pure bots as well. But it's difficult to do, and probably not something every developer is able/willing to develop or invest in.
Anything that makes assumptions about player's skills runs into problems too. For any online PvP game, the skill ceiling will rise with time. What once may have been considered improbable may soon become what's consistent for the top 1% or even 0.1% of the playerbase given a few years.
As well, it can run into problems as rebalancing occurs and new abilities are released.
But if the anti-cheat is able to advance to the point that a cheater can merely rise up the ranks by 10%, then, if you think about it... in a lot of ways the problem is solved. When I'm playing in a match, and one of the players is in the 80th percentile by their own merits, and another is "naturally" a 60th percentile player but is cheating their way up to an 80th percentile player somehow... and if they can't see through walls or insta-headshot across the map or do anything other blatently violating the rules, they just play a little better... what's the actual difference?
There is some. It's not zero. If you can't get those cheaters under control in tournament play the situation will normalize to everyone using a cheat just to keep up in a Red Queen's race, and that's still bad for other reasons.
But it isn't the same impact as playing with Sir Snipes-A-Lot who headshots you through three walls the instant your spawn invulnerability wears off, either.