One issue is the quality of the print. In my experience, quality control of the 16mm prints I used was very poor. The ones I've used were severely overcropped, poorly centered, hairs in the emulsion, had subtitles crossing frame boundaries, and missing frames. Older prints often experience varying degrees of color shift. And any print, including 35mm, is going to get scratched just from normal use. I think that if you want to make a really good transfer, you need to get a 35mm print that is as close to pristine as possible. Then you also need a very high quality scan.