Regarding the Necromancers – I just don't know, maybe it's because I'm not very good at playing as them. I just don't like that you can't properly replenish your troops in captured castles! Of course, skeletons recover and even increase in number after battles, but you can't get very far with just one skeleton:(
In general, regarding skills – I didn't say that Ballistics is completely useless! It's just not very useful. There are, in my opinion, six (or five) very useful skills that you should take, no matter what other options are offered: Logistics, Marksman, Leadership, Luck, Wisdom, and – on most maps – Pathfinder.
By the way, the Sorceresses have even worse skills than the Necromancers – they have a ton of Navigation skills, but it's only useful on a few maps:D
And there are skills that I would ban altogether to prevent the AI from becoming too weak: Eagle Eye and Diplomacy (it's terrible when all the neutrals practically stop running away and demand a huge fee for passage! The only good thing is that it's easier to get to Xanadu, as they are not encountered at every turn). You definitely shouldn't take Navigation – unless there is three times more water than land on the map.
Skills like Mysticism, Ballistics, and Treasurer – you can take them. Well, I already talked about the Treasurer separately in some post:)