No doubt you've read the many recent news items?
Tonga loses NZ aid over use of suspect Chinese aircraft - Australia Network News (Australian Broadcasting Corporation)
My thoughts are that the "MA-60 is unsafe" line is a smoke-screen which serves two purposes. It allows the NZ govt to:
- pull aid funding which would effectively be subsidising competition to a NZ company (Chathams), and
- avoid offending the Tongans by saying that Tonga doesn't have the capacity (skills, management, etc.) to run a safe airline.
As an expat living in Tonga you'd be fine as long as you don't miss fresh milk and good meat. I would be concerned about the competence of the airline management and maintenance, but if you can satisfy yourself as to those items I'd say it's worth checking out....