I hate to admit this, but I think Thessalians underpowered as well. Stat-wise Prodromoi offer the same charge value and better stamina at 75% of the cost. Thessalians do better at close-combat, but not that much, and it isn't what cavalry are meant for. The only use I can imagine for Thessalians is to engage and pin enemy heavy cavalry: here they can use their speed, armour and AP side-arm to best effect. However, they do suffer a lot of casualties in the process so you can only do this in a couple of battles before needing reinforcements. Hetairoi on the other hand do this almost without breaking a sweat, while Prodromoi won't do that much worse and are easier to replace.