Blitzkrieg is a sleeper commander.
I very much disagree, Blitzkrieg was never the most dominant commander at any time but probably always has been the (or tied with others) second best commander.
In ML5 it was the second most banned commander with 50% vetos for the very same reason you stated.
I would not say that panzer tactician is the best ability tho, it is always a very nice ability to have but not really a reason to pick a commander
The main selling point is the command p4 and ju87 but the strenght of specially this commander is that it has probably THE best combination of support abilities in the entire game that are all situational but very usefull if you need them. Recon, tac movement and tac smoke are (usually) all abilities that would not lead to a commander pick but you can always find situations where just the availability is the edge you need in a close game.
You can make a argument that this comander is never a bad pick, maybe not THE best for any stand-alone match but always a solid choice