Peltastai Makedonikai - A very good unit! I love this unit and I will defend their prowess. They have won a number of battles for me against elite non-phalanx infantry and cavalry.

Thueroperoi (spelling) - their role is to be cheap, but capable soldiers, like the Hastati of Rome. Expect them to take causalities, but if they die, at least they are cheap to replace. If that is not your attitude, then this unit is very disappointing (I certainly was initially).

Successor Medium Cavalry- like Thueroperoi on horserback. Cheap and die like flies, but widely recruitable - Any other expectation will lead to disappointment.

Thessalian Heavy Cav - I agree that they are not fantastic. I tend to recruit Greek Noble Cavalry instead if I have the local option. They suffer too many causalities in contrast to Companion Cav. You might be better off throwing away Successor medium units than investing in Thessalian. On the other hand, in the west (against Rome, Greece) they are pretty strong.

My disappointing units -

Median medium Cavalry - I thought that Media was supposed to be the place where the Persian Cavalry tradition flowered. These Cav are nothing but a clone of the generic "Asian" medium Cavalry, and they are weak compared to other Eastern Cav units.

Baktrian Horse Archers (I won't try to spell their Greek name) - This is a cool unit of armored Baktrian archer cavalry that that nation developed in response to the various nomadic threats that they faced. They are expensive and, and for the cost you can recruit lots of disposable Dahae riders instead.

Dahae Skirmisher Cavalry - Virtually useless - upgrade your MIC and get the riders instead.

Tarantine Elite Cavalry - Why do these guys have "elite" in their name? Am I missing something? The Greek armored Skirmisher Cav are generally better...

Spartiates - Solid killers, but they are expensive, and you have to have a level 5 MIC in Sparta to get them. In contrast, the Level 4 elite hoplites (I forget their name) that you get from the other Greek cities are simply better. That is what makes the Spartans (slightly) disappointing.

Galatian Wild men - Like the Spartans, they are disappointing only when compared to their peers. The Geseatae (spelling) are more powerful and have better morale. I was disappointed that the Galatians were not their equal.

East Coast Levies - From Arabia - they seem to match their opponents in stats, but their morale is so low that their inevitable rout after suffering 5% casualties will probably take the rest of your army with them. Use these guys for Garrison duty where they will have to fight to the death. Never take them on an offensive campaign.

Indo-Iranian Heavy Cavalry - these guys are portrayed as being one of the great Elite cavalry units in the game. Whoever controls Gandahara in Northern India can recruit them. They are supposed to mix the Equestrian tradition of the Steppe with the superior metallurgy of India to make an UBer Heavy Cav. They are very expensive, and very hard to come by. For all of that, I came away with the impression that the Indo-Iranian Med Cavalry was generally a superior force. They are more widely available, and have excellent stats for 1/2 the price. Once again, I found myself being disappointed in an elite unit when I compared them to their local peers.

Those are some controversial choices for "bad" units, but relative to my expectations of their capabilities, I found each of these units to be disappointing.