Most labs will develop&scan and then print from the scanned data. When a scan is made, they usually adjust the tonal range so that the whole gamut of the output medium is covered, so the results will look quite different from optical prints:
- With b&w you lose the degrees of freedom offered by different film developers and paper, as digital prints go to standard RA4 paper. Results will look more grainy than direct optical prints in my experience.
- With color negative film the tonal stretch/compression can save shots that are poorly exposed or have too high scene brightness, but will look a lot grainier than direct optical enlargement.
- With slide film you don't really have the commercial option of direct optical enlargement any more (), as Ilfochrome process is more or less gone, and I have not seen internegative based enlargements offered anywhere so far. The upside is that scans generally look ok.
I use scans as fast index prints and optical enlargements for the few shots that warrant the effort.