You are going to get a lot of mixed replies possibly as opinions will vary.
In "my" opinion they are safe in the reef and great to have. Why? Solid clean up crew. Easy to gauge nutrients based on population level. Usually only come out after hours. Work over sand, rocks, sump, overflows. Do not eat live animals be it coral or fish. I should clarify "healthy" corals and fish.
In general they are a good indication of system health and nutrients. They are part of the reef and natural cleanup crew and micro fauna management.