Easy, look at their stats and the role they're designed for. Compare how they actually perform in that role. I've used a lot of Western cavalry units in battles now, and I've found their heavies are almost uniformly rubbish compared to mediums or even lights. Always because their stamina is crap.
Thessalians are knackered (as in Tired) after a couple of charges. Which means any subsequent ones are nowhere near as powerful as the first two. Thrakian Prodromoi on the other hand (who have identical charge stats) can charge home like that five or six times before they're tired, and they recover their "wind" a lot faster.
More charges at full impact means much better as shock cavalry.
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