They are a single level 10 spellcaster, so yes you could cast all 10 spells but you have to roll focus after every spell and usually focuses are on 3+ or 4+, so if you fail that you're either burning Might or not casting anymore.
As for their combat ability - they get extremely slaughtered by heroes duelling them with Epic Strike (because The 9 don't have Epic Strike).
Also, their rules are better for normal formations. In fact it is usually a far better idea to just have 4 of them (for example Khamul, Betrayer, Dark Marshal and Knight of Umbar) than have all 9 wasted inside 1 formation. So competitively they're not very good (and neither is Sauron because Legolas can shoot him down in a turn... stupid GW
).