level 1 ultimate- sucks. only does 33% extra damage, highly doubt you have magic immunity so soon in game so very easily interrupted. cooldown is 2 minutes. mana cost is 100, so unless you have full mana, and you dont plan on spamming fury and other spells, its bad, seeing as he has low mana pool.
level 2 ultimate- meh. does 66% extra damage, you may have immunity, harder to interrupt, 4.5 second stun is ok. still cooldown is a lot, mana cost increaed by 2x, and abilities of the dark lady (wraith blades) and other heroes are like 10 times better than this.
level 3 ultimate- same as above pretty much, very high cooldown. 100% bonus damage is pretty nice, 4.5 second stun is pretty nice. of course level 4 wraith blades and other spells, like BH's damage increase, are better since they got by interrupted and they had added effects (silenced).
his canon jump kinda sucks. 2.25 second stun for a whole team is all right, if you can pulll it off well with your other teamates.
flick is nice early game, slows opponent and makes them very vulnerable to fury.
fury is great ability, except hard to use at times.
he starts out with a pretty high armor, except his stats are relatively low (i think) and his stats per level arent that great.
also, when using his ultimate, you are extremely vulnerable, while it pounds one opponent, you have to be near him to pound him (unlike succubus), and that means the rest of the enemy team can also pound on you.
i think that overall panda is a good hero with a good team, except without a good team that can follow up, he is bad, and unless you are really good with the flick +flurry combo, pushing the enemies to your allies, he isnt amazingly good.
i wouldnt say he is underpowered, except his ultimate needs some work.
None.