3 of my cats are indoor outdoor kids. About every other month we have collar disappearances. Over time, collars and ID tags become expensive to replace.

I wont be able to take two of my cats (Bob and Mynah) in for chipping for another month so I'm always worried that if someone looses a collar anyone will think they're adorable strays to take home. Is it common to use harnesses instead of collars? Is it more comfortable and safe? I know that at least with the safety collars those can break off during an incident.

I would not use a harness without supervision (attached to a lead).

Harnesses without supervision could lead to DEATH

I agree with the other posters. A harness on an outdoor cat could get caught on things, which could result in the cat possibly choking if the collar section was caught (depending on the type of harness) or could result in the cat being stuck somewhere and unable to escape, which could lead to them starving to death among other things.