For whatever reason, I never had any problems building, say, a Tomb Kings army or Empire army, but I just have this weight in my mind over building armies for AoS. Factions, sub-factions, units from multiple battletomes, campain books, boxed sets, GH, old warscrolls, replaced warscrolls, new warscrolls, formations, battalions ... decide if the exact same group of models gets one set of powers or another, add in an item to a certain number of guys...
It all just seems really, really convoluted. When AoS hit, it was simple. Put the models you like on the table and play with them. There are just too many sources and options now.
Don't get me wrong. I like to options. I like the ways to do things and the flexibility. I'm just saying that I have this 'fear' that I'll do it wrong and be told I've broken some sort of rule I didn't even know existed.
Edit: Just an anecdotal reference, look at spells. You get the two in the rules. Oh, and the one on your scroll. Oh yeah, also one from a book you may or may not own. Plus, there are all the ones that are on Warscrolls you don't even know you can have (the "all wizards know summon X in addition to their other spells" ones.