ditto makes mincemeat of most pure hyper offense teams, but it does little to nothing against stall or bulky offense. HO hasn't really been the top playstyle for a while and ditto is dead weight against too many teams for it to be popular. it has its uses, for sure, but those are very very limited when there aren't a lot of pure boosting sweepers around. the bulky ones (cm/bulk up) can beat ditto, so can any substitute users, and if you're gonna bring ditto in on non-boosting attackers, you could have just been using a more versatile scarfer that could do the same job, plus several others as well. ditto is far away from being the "best" revenge killer.
i think thundurus-T has the advantage over thundurus-I. they both outrun base 100s, which is the most important thing about their speed tier (admittedly incarnate also outruns stuff like gengar and terrakion), and volt absorb is way more useful than prankster twave (lol @ prankster nasty plot). that being said, it's not like they're completely incomparable. on the other hand i would say tornadus-T > tornadus-I by a pretty long margin.
and yeah most keldeo will get messed up by cm lati@s. hp ghost/ice doesn't hit hard enough and psyshock will send you to the grave even if you're a cm user yourself.