Surprisingly bad units

[Fondor_Yards]

I forgot about these guys before, the Iberi Curisi. They are classed as medium cavalry, but are on the low end of the stats to be medium cavalry, meaning most other medium cavalry and up will be able to beat them in a fight. So if they are going to attack anyone, need it to either be light cav/inf or outnumber the enemy. Almost all other light/medium cavalry is much more useful, either having missiles so they can harass the enemy flanks and break up their formation before chasing routers, or having lances so they can charge the enemy to rout them. The ones that don't, like the Curisi normally have higher stats so they can kill the lancers and missile guys, but have better stats. For the famous elite Iberian cavalry, they are sub-par at best.

[Andronikos]

Somebody here mentioned Brihentin. They are not exceptional, but I have very good experience with them. As the Roman cavalry is weak, Brihentin works wonders on them. I used them as Roman FM killers, when fighting on swords in cav vs. cav fights Romans fall like sheep (most of my enemies as a Celtic player were Roman armies). And when charging, Brihentin could rout enemy forces and are good against gaesatae. In the no-cataphract west, they are the best cav. aviable (have not tried German elites yet).

[Strategos Alexandros]

I forgot about these guys before, the Iberi Curisi. They are classed as medium cavalry, but are on the low end of the stats to be medium cavalry, meaning most other medium cavalry and up will be able to beat them in a fight. So if they are going to attack anyone, need it to either be light cav/inf or outnumber the enemy. Almost all other light/medium cavalry is much more useful, either having missiles so they can harass the enemy flanks and break up their formation before chasing routers, or having lances so they can charge the enemy to rout them. The ones that don't, like the Curisi normally have higher stats so they can kill the lancers and missile guys, but have better stats. For the famous elite Iberian cavalry, they are sub-par at best.

Curisi are actually quite a good light cavalry force for harrassing infantry and countering enemy light troops and skirmisher cavalry.

[Sdragon]

Lucanian Infantry
They have small shields and crappy armour so they die in droves. Only have 160 men per unit. Their unit description describes them as a peltest unit but they only carry 2 javelins. Every unit in Italy walks all over these guys.

[Fondor_Yards]

Curisi are actually quite a good light cavalry force for harrassing infantry and countering enemy light troops and skirmisher cavalry.

Exactly, as medium cavalry they fail, you might as well be using Iberi Equites Caetrati for that since they are cheaper and have javelins too.

[Disciple of Tacitus]

Jumping into the Iberian Curisi conversation - IF I were to peg one Iberian/Lusotannan unit as suprisingly weak for what it is, it would be these guys. They simply don't handle combat AFTER the charge well. For your money, Iberi Equites Caetrati are a better bet. The Iberian Curisi do - however - excel at countering enemy cav skirmishers.
I suppose the question always is - not which unit is bad per se - but have you (as General) figured out HOW to use that unit to the best of it's ability and - more pointedly - does it fit into your overall army.