I've logged hundreds of hours on Upwork as a remote worker and while every screenshot and all the keystrokes logged, etc, were relevant to the job it does completely prevent you from (for example) checking your normal inbox in case that gets screenshotted and you don't want your client to see that.
Okay maybe you don't want your time being used by the worker to check their email, or respond to a Facebook comment or pop onto Ycombinator for 5 minutes every hour but I just found it made me feel untrusted completely.
Working for well established clients that trust one another is the way to go, I personally use Toptal: https://www.toptal.com/#employ-only-on-the-ball-software-fre... and it can employ tracking if the client desires it for hourly jobs, but it's fully optional in part and full time jobs.
Though with Toptal you have a rigorous process for the freelancers to actually get into the system which no doubt solves much of the need for the additional screencaps as proof of work.