I use Thessalians heavily in my Epeiros campaign, and they don't tire too quickly at all, from my experience. However, I'm starting to replace them with Molosson Agema, as those guys have (very good stamina) and rarely tire. But don't beat on the Thessalians: they're strong, just not the best.

The Pantodapoi are good for one thing: Garrison duty. The Kavakaza Sparabara are better simply because of their superior traits, a longer spear, and because Hayasadan has to rely on them early on, and they can get the job done when supported by [great] eastern archers.

No single unit is terrible: it's all in context and use.