Mannfred is a very big based assassin. He can absolutely ruin the day of squishy models or units (expecially if he manages to cast something to get his buff on) and his casting and support abilities are very good too in the right armies. Alas he is very easy to kill for his point cost so keep that in mind.
Any army that sports a good number of units hitting and wounding on 3+ is a perfect fit for him so mass morghast in grand host, blood kights spam in soulblight and the such will be amazing for him. JUST BEWARE SHOOTING. He will die super quickly if he gets focussed by artillery and the such.
yes, he has to be the general in LoN wich is quite a bummer. In grand host he does not have to be, so it could be the better idea, there are some very good traits there.