BobCAD will work with STL files but I don't think you want to use them for mechanical parts.
STL files are best for art type work. The main reason is you can't edit the file, get wire frame from it or any other editing.
You can cut an STL and your can scale it up or down. But that is about it.
As far as file translators go with BobCAD:
DXF wire frame
IGES trimmed surface files
Step Solid files
BobCAD has other translators but this breaks down what works the best for importing into BobCAD
BobCAD does have direct file translators for autocad and solidworks and rhino, SAT, and X_T and B
But most of the time I use a STEP for solids and that seems to work the best.
Yes you can still have an issue with import because of bad surfaces poor drawings or un support surface types.
Sometime you need to clean up the file from import, or edit for machining.
Either way I feel BobCAD does a good job of this!