A lot depends on the transmission. In BMW land you can take apart the automatics and often get parts for them. The manuals rated higher for higher power (e.g. the Getrag 420G) you may be able to take it apart (even then you'll want specialized jigs), and you may be able to put it back together (with a monstrous press), but you still won't be able to find new parts for it.

Meanwhile you can take the automatics apart in your garage with few-to-no specialized tools.