Question about third-party Leica repairs

Curious to hear others’ experiences — if you send your Leica to a third-party repair shop (not Leica itself) like DAG or Steve’s Camera in Los Angeles, have you ever been concerned about parts being swapped out?

For example, say you have an M3, MP-4, M6, or M6 TTL — models where certain original parts are rare or no longer produced. Is there any risk that an original part could be pulled and replaced with an aftermarket one?

Is that something to actually worry about, or has anyone ever run into this issue?