Only thing I actually disliked is his involvement into the games. Although he did make appearances here and there, he wasn't much into the plot itself. If he were, like Steven, Wallace, Cynthia, Blue, and other champions of previous games, then it would have been a lot better, I believe.
Really? I'm gonna be honest, I actually thought he was THE most involved Champion in any of the plots to date, even moreso than Cynthia or Steven (and WAY more than Blue or Wallace, that's for sure). He does attempt to insight you on the bonds between humans and Pokémon despite what Team Plasma says, is strongly against their Pokémon liberation, and he follows you around a lot throughout while N is awakening his legendary, when you go through Dragonspiral Tower and the Relic Castle. Unlike those two, he even goes so far as to take on the main villain, N, even though he does end up wiping hard. For that matter, during the story he also gives you HM03 for Surf, like Cynthia does in DPPt, after you beat Cheren at Twist Mountain.
Not only do you not end up battling him until the post-game, but he also has a story to him in the sense that his Pokémon, Volcarona, died. That leaves him out of practice for a while and is what leads him to be defeated by N, but he also commends Cheren for his strength. And after you beat Ghetsis, he commends N saying that he felt inspired to get the legendary Pokémon because he wanted to, and not for Ghetsis. After that's over, you can find Alder in the Celestial Tower, where he talks a bit about his late Pokémon and how the player has got him to see more to what trainers have to offer before he returns to the Pokémon League.
As for battling him as the Champion, I don't know how easy or hard I can call him (not gonna discuss the whole "any Champion can be that" debate right now like I did with Iris), because it's been a long time - I think it was before BW2 even released in English, but that one time I had two Pokémon left, both of which were almost dead (I know one of them was a Honchkrow, I don't remember the other). Maybe I should take him on in my Red Mono-Color (without items and on Set mode) and see how hard he ends up being.
I'm gonna be honest, his character and story involvement alone makes him one of my favorite Champions. Would've loved to see more of him in the sequels (certainly helps that his grandson runs the Black Tower/White Treehollow in the post-game), and I do like Iris more, but I surely think Alder is a very memorable character.