Actually there are several adblockers available for Safari on iOS; the functionality was introduced in 2015. Adblock Plus and Adguard are some of the larger extensions available, and now uBlock Origin Lite is now being beta tested for Safari on iOS.
I never see these popups in Safari, so I think theory and practice is not the same.
I've never used these, but if I had to guess: these probably don't have the same power as full Manifest V2 extensions.
Also names like "Adblock Plus" scare me. I don't want someone I don't trust getting my web activity.
A quick Google search reveals AdGuard is only a DNS resolver, so it has the adblocking power of something like a Pi Hole. So... nowhere near as capable. It can't inject JS into webpages to prevent pop ups, which is going to lead to white boxes everywhere. In addition, ads can obfuscate the URL or share domains with non-ad content - so that content won't be blocked with a DNS resolver.
However, the software seems safe. Their privacy policy says they only store websites locally, and never upload them to servers. The app is also open-source.
Ad Block Plus is not privacy respecting. They collect usage data as well as unique device identifiers.
I find the "switch to Safari" talk amusing because the adblockers available for Safari are functionally equivalent to the MV3 API that everyone's complaining about. The problem with the "static list of content to block" approach that Safari and MV3 use is that you can't trick the site into thinking that ads have been loaded when they haven't, like MV2 allows via Javascript injection. The effect of this is that you'll run into a lot of "disable your ad blocker to continue" pop-ups when using an adblocker with Safari, while you won't see them at all when using an adblocker with Firefox.