With the APs you can set them up in standalone mode without the need for a controller at all, but you get more out of them with a controller so if you have the means to run a controller you absolutely should. With the controller you're heavily incentivised to sign in, but you don't have to.
With their "Cloud Gateways" (e.g. UDM Pro, UDR, UX, UCG-Ultra) it used to be difficult if not impossible to get them into a usable state without signing into a UI account, but that is no longer the case since a few years ago.
There are some exceptions, if you wanna use their cameras you lose out on some features if you don't sign into the cloud to activate those, specifically facial recognition stuff.
All I want is a standalone AP. I don't care about advanced administration, I just want it to connect my wifi using devices to the rest of my network.
I switched from consumer to Unifi just because of the better signal.
Unfortunately, even in just this thread there are very mixed answers. One yes you can (and a downvote that probably means the same thing), your 'maybe' and one 'i couldn't'.
OK so I just setup a blank controller (version 8.2.93, latest stable), no UI account (you just click "Advanced setup" and then skip when it tries to get you to supply an account), I then adopted a factory reset UniFi AP AC HD I had laying around, it accepted it without complaints, and automatically updated it to the latest firmware available (this is an option that's on by default in the controller software).
I then factory reset the AP AC HD again, and set it up as a standalone AP using the Android app on a freshly reset Pixel 6 running the latest version of Graphene OS.
Both methods worked fine to setup the AP without any UI accounts involved.
This is available on all APs able to run the current version of the AP firmware, which is literally all the APs released since and including the AP AC Lite (which is from 2014 or thereabouts, they support their devices for a very long period).
7 Pro Max is the current top-end model.
I highly recommend setting up a controller for managing the AP though, even if you don't keep it running 24/7 (you can simply start it on any PC whenever you want to make configuration changes), as when they're in standalone mode a lot of the cool features don't work, you only get one SSID (per radio) and no roaming support for example, whereas when managed by the controller you can have up to 8 SSIDs per radio on the latest APs, you can access telemetry, setup roaming, etc.
I honestly can't recall that ever being true for the controller, but it absolutely was true for the UDM/UDM-Pro on release and for quite a while after, so I guess it might have been true for the controller as well for a while around then too, I don't setup controllers that often.
Maybe it was just hidden, like it is now, but it pissed me off enough to not research it.
> I don't setup controllers that often
I've only done it twice; once when I installed the AP the first time, and once when I retired the machine with the controller so I needed to install it on a new desktop. Second time is the one where I failed by refusing to make an UI account.
With their "Cloud Gateways" (e.g. UDM Pro, UDR, UX, UCG-Ultra) it used to be difficult if not impossible to get them into a usable state without signing into a UI account, but that is no longer the case since a few years ago.
There are some exceptions, if you wanna use their cameras you lose out on some features if you don't sign into the cloud to activate those, specifically facial recognition stuff.