I got tired of getting my mods from two sources, and I wanted to be able to choose when to update mods so I unsubscribed from all my Steam mods and downloaded them from Nexus, where before I had some from both places. Ever since then, I have not had a single CTD with the same set of mods I had before, on my new character or my old character. This includes mods like SoS, and I can run multiple quest mods without issue. With the Steam mods, I would CTD regularly with certain mods enabled.

I'm thinking this has to do with ESMs being more reliable than ESPs? Or is it the other way around? Just wanted to pass on this info...

Do you use the nexus mod manager? It might of been your load order because NMM uses BOSS to order it.

edit: Well I guess it would still modify the load order of the steam mods, so I guess not but I use a few steam mods without trouble.