If one does not merge Ancestry.com Census, Death, Birth, Military records, etc., then you may not have issues. I have full control which of these types of records are added.
As noted above, I can create a 1 person tree and make a smart story of the person's Revolutionary Military records and cause FTM 2012 to crash from out of memory issues.
If you exclude the media and sources, FTM version 16 is a much better program speedwise and freedom from crashing-wise. Plus, the file can be up to 2GB in size.
Edit: The issues of speed etal, can be easily fixed by the programmers if they take time to do so. If 1994 era FoxPro can handle large files, so can FTM.