I use Safari on iOS for obvious reasons and the inability to export the bookmarks I have on my Mac browser to Safari is really annoying. Fun fact: if you have a Windows computer you can do it https://support.apple.com/en-us/HT203519
What obvious reasons? Why not just use Firefox for iOS instead?
One of the main reasons I wanted to use it was to pass pages from desktop to Firefox for iOS, but on top of start time it would take upwards of 20-30s just for the tabs to sync across devices.
What I would notice is constantly having to look up my sync-data like history and passwords from elsewhere because Safari is a walled garden. That would seriously hinder me and slow me down on a quite so regular basis.
Different folks, different strokes, I guess.
You cannot set another browser as the default one on iOS, that's the main one.
Yes, having to copy-paste URLs from Safari to Firefox sometimes is cumbersome, but it's a million times better than having to use Safari as my main browser.