Earlier quoted context omitted.
Ah! Oops! I meant break things FROM debian, as in got code from Debian and broke it in Ubuntu, or didn't pull fixes from Debian into Ubuntu for years. My apologies! I'm not a native English speaker :( and sometimes I think I am smarter than I am :)
What did they break?
they broke the initscripts so array assembly didn't work correctly - not even the same as the installed man page stated
they had mdadm version 2.x (has significant flaws) for YEARS after mdadm 3.x was accepted into debian - YEARS - even after multiple users making clear what the problem was in bug reports
a user even made a patch and a ppa which fixed things but still they didn't pick up on that for a year + as i recall
and they broke/removed debian's PERFECT support for installation onto md raid
other stuff i've honestly forgotten but i swear there was more
oh! oh! wait, also the STOPPING THE BOOT PROCESS TO WAIT FOR YOU TO PRESS A KEY if a drive needed an fsck!!!!! what the f....
(don't understand why you were downvoted, good question)