Pretty much any of them aren't too hard if you have a doner vehicle with all the parts you need. Just take all the stuff out of the doner and transplant it into yours. I've only did this one Volvo but I have done several domestic "muscle cars". The hardest thing about the Volvo was getting the pedal assembly bolted back in. Gotta turn into a pretzel under the dash, can't see the top two bolts, no clearance, etc.

Bimmer shouldn't be any different than any others, I've changed clutches in a couple but no swapping... About the only monkey wrench I could think of would be if a manufacturer were to put different cranks in and you didn't have a hole to put the pilot bearing in!

Justin