Maybe over my time of playing EB I've had a different EDU set, but the fact that Thessalians are getting ragged on is odd to me. They WILL beat Traikioi Prodromoi 1:1, I can attest to that, but it will hurt. Both units are badass, in any case. Thessalians also can last a long time in melee and will break no/low-armor missile units on their own if you choose to waste them for such. I loved using Thessies as FM killers, esp once they chevroned. Since you shouldn't be charging any elite infantry with cav short of catas IMO (though lancer types like Prodromoi do for a poor man's solution) the Thessies should be more then able to hold in a melee against the medium and lesser infantry, esp if they do a proper flank assault.
As for slingers, love 'em, but I don't worry about them as long as I have a balanced army. Either I'll get some light cav around to slice and dice or I will pound them with my protedcted archers (not slingers) and let my heavy inf take the occasional casualty from slinger fire. MY slingers target heavy cav/FM, heavy inf, and HA's in that order. Unless I flank with the slingers phalanxes and large shields are generally avoided. Love getting them behind phalanxes though.
I can't say as I've ever met a truly surprisingly bad unit, though some have not lived up to the hype I read on the forums. Thraikioi Peltasti come to mind, as do Gaesetae, which are frankly NOT THAT HARD TO KILL if you are smart about it. The Thraikes, well, the hype made them to be an uber-peltast. As I found it to be, they are better than the standard variety, but not by that much. In fact, they seemed to take cavalry charges worse than normal peltasts (yes, i know, shouldn't be letting it happen but sometimes you just gotta line 'em up and take the licks when you haven't had the chance to refill the army) though they are a notch above in melee ability. Nothing to write home about but i do recruit them in both regular and merc form when I in those areas. I just prefer the standard peltast's vast recruitment area.
And that is the best part of EB; you can call a unit bad if you want but that's mainly because it doesn't fit your generalship style. And those badass units tend to be hard to recruit whereas main units are wide in area. It all balances out. I will go with the anti-elephant crowd though. Even the armored ones don't faze me since I'll just rout the rest of their army first and gang bang the eles. And Velites are just plain depressing for me; I'll hire peltasts instead.