I have not used simulation express yet, nor know anything about it. My coworkers have checked various models and have particular guidelines on how things are modeled, so that analysis can be done (so they say). I can't remember all the specifics as far as what those guidelines are, since the analysis is not done very often. I know they are against using assemblies for a lot of things and prefer to show weldments whenever possible. However it seems like when I create a weldment, there are still complaints...mirrored bodies are bad for them, and a few other things. All I hear is "I had to recreate your model so that I could do this analysis, etc.." I wish I could offer more details, but like I said, it isn't required all that often.
Seems to me that a solid body is a solid body whether it is parameterized or a dumb solid, or an assembly. I design things so that it makes sense to me on how things are assembled or welded, while allowing for future changes if possible. Are they just not informed on all the tricks about simulation stuff?