Oh, I quite agree. Have been around here for the upgrade from 17 to 18 which if I'm not mistaken was in fact the first time a major-version upgrade was offered in the first place so problems forgivable and moreover not used too much yet, from 18 to 19, a bit less so and many issues there were, and now from 19 to 20 which as far as the general impression from reading this forum goes is a bit of a disaster, and one now combined with the to 19 new System report thingy advertising it to unsuspecting users.
The issue is of course that the Mint team would/will not have expected serious issues --- and frankly most I've seen also seem to occur as a matter of even Mint 19 having been upgraded from 18, i.e., old gunk in the sense of cgmanager or KDE stuff still laying around which have in those two years gone from gracefully failing to simply bombing out. Well, yes, you can't test everything and given my views on people in-place upgrading between major versions in the first place I find it hard to feel sorry for them, but sure, still not a good idea to advertise an upgrade to these users.
But that said I do not feel it in any way likely that your BIOS issue was anything specific to the upgrade from 19 to 20 as such. We have seen similar behaviour here on the forum of no longer being able to boot from USB and/or being being all of a sudden unable to enter the BIOS setup as a matter of buggy early UEFI implementations. Haven't in fact dug through this thread to see if you have specified enough of your system to be able to confirm/deny that possibility but there really seems to be no other option than quite system-specific trouble as to that issue you had. Again, you can't test everything and certainly not every system out there, and if indeed we're talking about that rare issue involving installing any OS in UEFI mode on some systems, well, you'd just be out of luck there I'd feel: would not feel the update to be responsible in that case, even if it would be in a practical sense.
But otherwise then, yes. I wish Mint would go back to not supporting major version upgrades, certainly back to not advertising them. Way too much trouble with them.