I daily use complex multi assembly parts both from Catia and solidworks exported as STEP files. Altium 13,14,15,16 have never barfed at any of them and these assemblies can have hundreds of sub-parts (plastic covers, gaskets, screws, heatsinks, coils , busbars, you name it. Step files in the order of 10 to sometimes 30 megabyte )
I call inventor problem. Made by the same company responsable for that other disaster : the DXP file format.