I used to do a lot of part modelling and making assemblies from it but it is slow and a lot of work to maintain relations that way. Making sketch or feature relations in an assembly as a part is a bad idea if you ever use that part in another assembly. Making parts internally to an assembly means they can only ever really be used for that (or has high potential to go crazy with missing references at a future date). I have mostly used multi-body part modelling for a while because as long as you really understand the history tree and parametric features and relationships a well-designed model shouldn’t break features if you change one at a later date. I think the ideal solution is using multi-body parts as a master model for overall forms broken down into multiple parts for manufacture and then using assembly to bring in all other fixtures and standard components.