Sigh. I specifically referred readers to the Crawdaunt thread for this, where Alexwolf and others argued at length to demonstrate that Crawdaunt is not outclassed by Azumarill. Basically Azumarill is capable of being that offensive pivot like you say because of its defensive capabilities, but Crawdaunt is a stronger offensive presence (and not insignificantly so). The two are played differently and have significantly different capabilities. Azumarill may be better overall, but it does not outclass Crawdaunt.Crawdaunt is a pretty cool pokemon, but its sorely outclassed by azumarill, who can actually switch-in on shit and function as an offensive pivot. Crawdaunt cannot switch-in on anything and relying on just aqua jet is really dumb without rain. Water/Fairy is such an excellent typing that Water/Dark cannot even begin to compare to. I'll just leave it at that.
The argument for M-Abomasnow is weaker than Crawdaunt's (possibly since I've used it less and am therefore a bit less confident in its abilities) but the issues you highlight seem overstated. The three switch ins you mentioned are as good as they get for Abomasnow (you'd have to tell me about the "many possible" other ones, because when I think about it and look through the list I'm not seeing them) and even they cannot switch in more than twice (Heatran can't switch in at all with correct prediction, therefore it's not strictly a switch in at all in this context). If that's the best the tier can do to defend itself I think my statement that he is "virtually impossible" to switch into safely is vindicated. Genesect also makes just about everything lose momentum. It's why he's being suspected.Mega Obamasnow is a sad case. I used him in a TR team in OU once, and no matter how hard I tried to specialize mega obamasnow into a team, it only peaked above 2000 once. This is because of his amazing amount of weaknesses, which includes rocks, brave bird, mach punch, bullet punch, and u-turn. It brings so many weaknesses to a team it most certainly needs more support than simply hazard control. It's true the not a lot can switch-in to it's great moves, but a couple come into mind:
1) Genesect can take any one hit and threaten you out with everything, making you lose momentum.
2) Heatran can come in on both STABs and kill you before you can eq. Earthquaking on the switch is the only way to go, and if you risk it with the pokemon that is currently in, chances are that its offensive and you're dead.
3) Aegislash is a big problem, with tank-slash taking less than 50% from eq and retaliating for huge damage with shadow ball or iron head.
That's just many of the possible switch-ins that's easy to abuse him. His "bulk" means that you will have to invest a large amount of your evs into hp, meaning your earthquakes are even weaker as your switch-ins grow. Mega obamasnow is just not a pokemon meant for this tier.
As for the threatening moves you listed: Brave Bird is a Talonflame threat pretty much exclusively, and yes Talonflame revenges. But it can't switch in (252+ SpA Mega Abomasnow Blizzard vs. 4 HP / 0 SpD Talonflame: 141-166 (91.5 - 107.7%) -- 50% chance to OHKO). Talonflame revenges a lot. That's not really a con.
Mach Punch and Bullet Punch are also small issues. Abomasnow is so slow that priority is only something to consider when you are trying to pick something off with Ice Shard. And they are so weak that they can't do too much anyway. Here's the scariest Mach Punch: 252+ Atk Conkeldurr Mach Punch vs. 252 HP / 0- Def Mega Abomasnow: 86-104 (43.6 - 52.7%) -- 17.6% chance to 2HKO.
As for U-turn, you just have no right switching into a U-turner. If they have to switch into you, however, they take a hard lash. If you have hazards up they are going to be really shaken up by the whole thing.
None of the threats you mentioned are devastating to Mega Abomasnow's viability in the rank I suggested for it:
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.