i think it more has to do with a team running these things when theyve had time to set up. attacking those classes even with an uber advantage can be tough if you don't know what you're walking into. last season gank and the meisters did this same thing and were able to beat a team i think they would have had a hard time beating on a 5cp map.
the reason that OSB (the team gank was playing) lost was mostly because they didnt know what to do. instead of adapting, they ended up just losing because they didnt have anything that could really counter those tactics. i'm not saying that a team with good teamwork or communication wouldnt be able to overcome those obstacles, i'm just saying it has worked in open before.
all of the things you mentioned in regards to the flank and heal spread is pretty spot on, too. in fact, i would think that most teams at this level wouldnt even realize those weaknesses and try to exploit them, hence feeding the misconception that running heavy/pyro/sniper/spy or whatever on viaduct is hard to counter.