I ran into one issue with XnViewMP. The TIFF with alpha transparency I converted from a PNG transparency works fine in Pro+, but when I sent it to my editor who is on Mac the background came in as black. So I think there may be some Mac v Windows issues. Photopea's conversions worked fine.

However, Photopea does not have the ability to create Grayscale images. XnViewMP does have this ability. BUT cannot change modes to CMYK. So, until I have a greater need for something like Affinity, I'll use both.