[Baalz]

Well, I think I've been clear on why I think can be powerful.

1) They've got access to powerful (3+ level) mages in astral, nature, earth, water, and air, leaving only death, blood and fire which take some work. They've got good/great national summons in death & blood, so a little effort in diversifying goes a long way. This leaves them really only 'weak' in fire, which is arguably the least useful path. Head of the class in diversity.

2) Some of the best non-unique thug/SCs in the game at fairly cheap prices. For 25-35 each they get a wide variety of guys who are not only good thugs, but also powerful combat and forging mages. On the high end you can field almost 3 Rudra for each GOR'ed Seraph, and all your guys are sacred.

3) Very good non-commander summons ranging from cheap ethereal chaff to uber heavy infantry.

4) Lots of 'special forces' type recruits, from knight slaying Markatas to stealth archers to stick and stone flinging Bandars to elephants (who can be great even into late game if your opponent wasn't expecting them).

5) Your most expensive (and best) stuff all costs pearls so alchemizing from anything is an option most of the game. Most of your stuff can still be cost effective at twice the cost in an off gem.

Ways in which Bandar can struggle

1) No single easily accessible 'use almost anywhere' unit. You can't crank out your 'best' unit, confident they'll get put to good use against most any opponent.

2) Lots of expensive options, lost of chances to waste money and gems on suboptimal choices. From summoning a hundred gandaharvas to massing thirty elephants to fielding a swarm of Bandar longbows, you've got a whole lot of opportunities to waste a whole lot of resources on expensive things. As I was discussing above you have to be willing to use the Apsaras if you can get away with it, even if you could summon gandaharvas at the moment. You have to be willing to turn back those F/W vans using markatas rather than spending 4 times as much on Bandar troops.

3) Several "gotcha" weaknesses from generally low moral to having the animal flag to having map move one recruitable mages to having only H1 priests. The reason I describe them as that is because none of them are crippling...until you forgot about it the turn it mattered.

[edit after thinking for a moment and AoE's comment]
4) Several "necessary" things that you really need to spend your gems on. Compounds the expensive options problem. If you can't clear a minimum hurdle for gem expenditure you're pretty much hosed.

Basically, Bandar gives you a whole lot of chances to shoot yourself in the foot. Played by a very experience player I think they'll fairly consistently do pretty well - even against opponents of equal skill. Played by even a moderately experience player I think they're pretty much always going to struggle to be competitive.