Well, if we really want to answer this, let's talk about terminology I will use:
- Weak means a commander who in the majority of situations is an inferior choice, and between two players of equal skill, will lead to the opposing player winning a majority or even supermajority of the time.
- Average means that all else equal, the commander doesn't shift the balance of power, and between two players of equal skill, will lead to the opposing player winning half-ish of the time.
- Strong means that all else equal, the commander does shift the balance of power for appropriate lists, and between two players of equal skill, will lead to the opposing player winning less than half of the time, but not dramatically less.
- Overpowered means that all else equal, the commander dramatically alters the game, and between two players of equal skill, will lead to the opposing player losing a supermajority of the time barring massive luck.
So now Ackbar.
I think we can immediately dismiss that Ackbar is weak (plenty of people are winning consistently with him), and given the strong rebel preference for him in the Vassal tournament so far and in local play, I think we can go so far as to say he's probably even a cut above average.
So now we need to ask: are there counters to Ackbar, or is he overpowered? I will say that having played both with and against him, what makes him strong is that his fleets are easy to understand, easy to use, and bring a mathematical advantage (more dice) that cannot be screwed up unless a player is really poor. In other words, Ackbar is going to be strong in most situations. However, he does have some flaws:
- He does nothing for squadrons, so against a strong squadron build, unless you can use his ships to kill the carriers, you are at a disadvantage (e.g. boosted comms Rhymerball)
- He does nothing if you are out of position, so against faster fleets (or those with a synergy between movement and objectives), you are at a disadvantage.
I think this explains why a Raider Rush list (or similar Gencon Special type thinking) could do a number on an Ackbar list. If you jam the line in the face, they have no advantage and are getting pounded. I think this also explains why Rhymerball is a thing again. I think it also means that the CR90 list could be a nightmare for Ackbar.
Either way, I've seen him lose. It's not like you can't beat him. It is, however, like you can't beat him if you didn't bring tools to defeat strong lists and understand your own list. Based on this, I would rate him strong (certainly the strongest of the Rebel admirals in a vacuum, though for purpose built lists, I think MM and Rieekan can be very good), but not overpowered (he is going to be common, but he's not gamebreaking).

