Mastering in Sonar, why not?

This must have been disussed before, but still: is there really a major difference between mastering a mix in Sonar track and using a costly, separate mastering software? Honestly, for a decent demo, is there something essential missing, if you import the mixed wav into a Sonar track, equalize it, compress it with multiband, do other plugin-based editing, clean the start, process afada, etc.? Please tell me the big difference, and I'll be saving for mastering tools...