Some are very obvious, yet they are always "already under investigation".
Take a quick example:
Warframe cannot be completed to 100%, because the content to get the last two mastery ranks has not yet been released(*). Yet on this site you can find some people that are so skilled that managed to get those ranks, 29 and 30, even if the content is not yet there. That should count as an achievement in itself :D .
Seriously, an automated check on unattainable achievements would start removing the most obvious cheaters.
(*)Here are some external links to verify:
https://warframe.fandom.com/wiki/Mastery_Rankhttps://warframe.today/warframe-mastery-rank-test-guide/