Just got back from a trip to Myanmar and used RJ to Bangkok (good price for Bus Class return*) then Myanmar Airways International for BKK-RGN. MAI service was fine (only a one hour** flight) and they seem to have a modern fleet and good safety record. For my return flight, they managed to check my baggage right through back to London and both they and RJ in BKK seemed to be very used to dealing with this situation (not just MAI but also other non-Oneworld airlines connecting to the RJ flight from BKK).
*which also counts as four long-haul legs, so 480TP
**BTW, Myanmar has a half hour time difference from Thailand, which makes flight times look a bit weird.