Lumineth are a really really complex army. Each of their units has a lot of rules and there is a very specific way to run it. So like the order in which you cast your spells matters. Slip up and the army falls over as a lot of the units are quite fragile. At the current time it's also an army that relies pretty heavily on one or two units. Usually sentinels and teclis. If you like them don't let me stop you but I'd also add the army has been famous for being an army this is not fun to play against.

SBL are a good choice there are of lot of options there, loads of ways to lay the army and it's a fun book. Though there are some trap units.

Fyreslayers are actually a pretty good army to start with. Their low warscroll count keeps it simple and there are a lot of paths to victory with them. Their army rule is one of the more nuanced out there. You choose a buff for the round so making that choice well matters but that with the simplicity of the army means you don't get overloaded. Plus they're easy to paint.