Managers aren’t worried about the remote workers who are productive and reliable. The worry is the people who aren’t. On my team, you are fully remote as long as you meet expectations. You don’t, you return to office.

Fine, but that mean that they have no way of measuring productivity other than the "I see him doing his work" or "I see him at his desk" methods.

They only mention it once, but I do have issues with mentorship in a remote work environment. I just personally haven’t been able to make it work. I’m sure some do.

This is a minor problem. You can implement a progressive WFH policy where the new hires must be in the office with their menthor for the initial training period and then begin to WFH for more and more days. The downside is that the company need to return to hire locally which could means to pay the new hires higher salaries.