Magic is quite powerful in Disgaea 2 and Adell doesn't have the Intelligence or Staff aptitude to make the most of it. That said, Adell's passive has carried me a few times (such as the Z Pirates, Baal, and Prinny Baal) and Fists are a pretty good weapon type for several Dark World stages. Most units are viable in various endgame situations and Adell is no different. My main concern with Adell is that his passive becomes useless over time, but his high Attack and Speed help mitigate that when you hit that point.

With 25 Adell Lover, 100 Professional, and 100 Firefighter Specialists, 255 Fist mastery, and a Level 99 Big Bang, I'm hitting for about 3 to 4 million against enemies with +50% Fire resistance and I'm nowhere-near-finished grinding his Reincarnation levels.

I guess it depends on what you consider "viable"? As Kraleck mentioned, magic is quite more powerful (and flexible/versatile) than physical attacks in general, though that doesn't mean you can't use the later to achieve good results. While some characters are better than others, IMHO you can make any character good with the proper equipment(s).

Just as a sidenote, @Kraleck, elemental affinities (and the respective specialists) only increase dmg with spells (not skills in general), which is one of the reasons magic is so OP in this game.

I noticed that when testing using Peta Fire VS Big Bang as Adell, neemoos. I decided to teach Adell Peta Fire for a ranged option (the Firefighter specialists bring it up to decent damage levels for him), all 4 tiers of Heal and Espoir for self-preservation (Adell was the last unit standing VS Prinny Baal when I fought him the first time), and Braveheart to help with level grinding other units (was quite useful when grinding the first few hundred levels on LoC CoO4 for fresh units).

I mean literally everything is viable in Disgaea if you put the time into it.

Laharl is the strongest unit you have in Disgaea 1, so that's a weird take. Some late-game classes have nice abilities but their damage output isn't really going to be different. If you're comparing physical fighters with magic users, magic users always scale more quickly than physical attackers in all of the Disgaea games, especially when you exploit elemental weaknesses.

Eventually everyone's hitting the damage cap though... so I don't see how anyone can argue that there's such thing as viability argument in Disgaea.