Hi Andre,

re:
> Is there anything wrong with using joined planar faces to build a solid?

Yes, if you want to fillet it - the filleting engine will usually get confused with planes being split up into multiple coplanar fragments instead of one large plane.

One way to clean up those areas is to delete the fragments then select the object and use Construct > Planar which will build one large plane face there and join it in.

- Michael