Hey, I own one of them. (E88 135i).

It's actually a convenient location for the battery, especially since there are battery posts under the hood so you can charge the battery if it's totally flat without trying to get into the electronically operated boot.

... The only thing is: NEVER shut the boot with the battery disconnected, because if you try putting a power supply on the under-hood battery posts and it doesn't work because the battery is disconnected (which is not uncommon) - then the way in is as convoluted as this tesla process.

Anybody who's worked on a BMW knows to put a towel or floor mat over the latch before disconnecting the battery, then leaving it there until the battery is reconnected.

Edit: That said, I recently had to replace the battery in a E53 X5, which involved this very simple process:

1. Put down the back seats and climb through to manually open the rear hatch.
2. Move all the crap stored in the back of your X5 out so you can get to the floor.
3. Pull up the floor panel and find something to hold it in place because it doesn't come out completely and most cars have their little BMW roof-hook broken by now - so this floor mat will bonk you on the head a couple of times while you execute the remaining steps.
4. Unscrew and remove the spare tire.
5. Unscrew and remove the steel structure made up of three separate parts that holds the spare tire in place above the battery.
6. Remove the battery.
7. Insert the new battery.
8. Reverse steps 1 - 5 to put everything back together.

BMW's might be fun to drive, but they're not fun to maintain.