Anyone else find finding unique monthly visitors for a website harder than the guides suggest?

The guides skip the part where every number is an estimate. For someone else's site you're looking at panel and clickstream guesses, and I've seen the same site come out two or three times apart between tools. Use them to compare sites against each other, not as a real visitor count. For your own site GA4 "users" isn't truly unique people either. Same person on phone and laptop counts twice, and anyone who declines cookies may not be counted at all.