Adamant Feraligatr 2HKOs Suicune with +2 Crunch, though. Pidgeot can't do anything like that outside of Hidden Power Water (or another HP that Aero is weak to). Mega Aero keeps Mega Pidgeot out of S because of 1) How ubiquitous Aerodactyl is, so using Pidgeot is instantly a liability and 2) How completely Aerodactyl shuts down Pidgeot. Suicune is a significant roadblock for Feraligatr, but it is not the same level of counter. Suicune isn't as popular, and Feraligatr also has the SubDD set, which sets up on non-Roar Suicune and then sweeps, instead turning Suicune into a liability.
I don't know if I'd put Feraligatr in S yet (at least, no in standard, he's definitely S in No Scald, where suddenly every bulky Water is set-up fodder), but Suicune doesn't auto-disqualify him for S. None of our S Pokemon exactly live up to the "free turn/zero drawback" standard of the description (which I think is a sign of the balance in the tier), but they are head-and-shoulders above the competition in sheer utility/danger, and while I've played around with Feraligatr a fair amount I think some more testing is in order as to whether or not he belongs there. Unless Aim and whoever else is responsible thinks otherwise.