He will probably be better in any army except SoB. Because of he will cost more than 600 points (and I think he will) you have only one Mega, Kragnos and one unit of 3 Mancrusher. Maybe some points for another unit of maybe 1-2 Mancrusher left. Mancrusher are made of wet paper and die very fast, so you have only one Mega and Kragnos. Depends on the battleplan and enemy but you are very easy to outmaneuver.
Another reason is that Kragnos doesnβt benefit from the alliance rules and he counts as one model too.
So he is the best in a Orruk Warclan list or maybe Grand Alliance.
So we talk about the Grand Alliance. Megas in the Grand Alliance are much better than in SoB. A few weeks ago I easily smashed a SoB with 2 Megas, Gordrakk (for buffing) and 4 units of Savage Orruks. My Megas killed his Megas and I had more units for multiple scoring. Easy mateπ