Keep in mind that empire isn't a finished nation yet, and due to the whole "every mage that's worthwhile costs gems" mechanic it may have some fine tuning to do yet. But before the most recent version, empire has felt on the weak side if anything.
So the latest version of empire has fairly cheap level 2 mages in all paths save blood now. This is pretty much the big thing this nation has going for it, cheap diversity. The recruitable mages are absolutely useless except for research and summoning your real mages. So while you do get a lot of diversity, you are spending gems to have your mages that you can't spend elsewhere. So while it is cheap to get diversity, and a lot cheaper than many other nations get as far as access to non-national paths goes, this nation does have to keep investing because all of its mages are diversity mages. It is the main strength of the nation though.
On the other hand its army is hardly awesome. It's pretty mediocre, in fact. Pretty much anything you can recruit is more or less the bog standard of average, with only a few standout units. Marksmen due to being the cheapest and most massable arbalest-wielding guys ever are the biggest of these. Nothing else is really better than usable. Swordsmen are good arrow catchers, and their cavalry is decent. Tribute to order summons quite nice cavalry units, but they're gold inefficient compared to the recruitable knights, with the exception of the inner circle reiksguard. And flagellants are those same guys marignon gets, and that you get from random events, with the exception of a few more hp. Which, while they do work, are still not that amazing. The big benefit of them is that they're very, very cheap. Even greatswords aren't amazing, you really need the retinues to make them justifiable because they recruit/move so slow.
I definitely don't think altdorf is the strongest of the warhammer mods right now. I'm not sure who I quite think is, but there are several contenders. I had the luxury of playing Itza before they got nerfed hard, and would have had no trouble saying they were the strongest in the past
Maybe they still are, but they've lost a lot of their potency lately.