[kup0]

Have used a Pi-hole for two years and maybe had to whitelist one or two sites (easily found looking at the log), which was easy using Pi-hole's web interface

Additionally, you can simply disable Pi-hole for temporary timeframes using the web interface as well, it has options like "Disable for 15 minutes", if you don't want to bother adding things to a whitelist.

I've used all sorts of financial, ordering, etc. sites and generally have not run into an issue. A single credit card site was one of the two I had to whitelist, and it was easy.