[Google Pixel 6 Pro] roirraW "edor" ehT:
My wife and I had the Note 10+, not the S21 Ultra, so I can't speak to that.

For the Note 10+, complaints were:

Slow ultrasonic in-screen fingerprint reader. Some folks have the same complaint about the P6P's optical in-screen, but it's worked very, very well for me.

Samsung's duplication of multiple services that Google supplies. You have to know what you're looking for and jump through all the proper hoops/settings to set defaults back to Google's solutions since that's what I prefer.

Related to Samsung's duplication of services/apps, their phone app duplicated a bunch of my contacts (and even tripled some) in a way that Google doesn't recognize them as duplicates, so I'd have to go through manually, compare, and delete or consolidate contacts.

How customization of the active Quick Setting tiles was set was really frustrating, up through and including Android 11 on the Note 10+. When it gave a list of the QS Tiles I had to choose from (to add to those that normally show), it presented them in a single horizontal scrolling row across the top of the screen only and in no order that made sense to this human, instead of Google's method of displaying all the tiles available in all of the screen real estate that's available, so even if the order the "inactive" ones doesn't make sense, at least I can see 14 of them all at once and only have a little over two pages of them instead of 8 pages of single-row QS tiles to choose what to activate. This is particularly frustrating when using third-party apps to enable better performing QS tiles for WiFi or Mobile Data toggling or other purposes, so I would have more than one nearly identical-looking toggle that would get confusing to choose from, and from what I remember, the custom third-party app-created QS tiles weren't necessarily together or at the very end of the scrolling list.

Despite disabling all background snoozing for particular apps like GMail, Hangouts/Chat, etc, I couldn't get timely notifications of emails and chat. This was particularly troublesome for work, which also uses Google for everything. I opt not to have a work-supplied phone as it would be iOS. As an aside, my coworkers who do have iPhones from work also have trouble with some chat notifications not being obvious or immediate or some such, so that wouldn't necessarily be a solution anyway.

Before the Note 10+ (on the Pixel 1 I used before it), my emails and chat notifications came through immediately. I sometimes got email notifications for work several hours after. I do enough multi-tasking and when I'm very busy I just don't get a chance to flip back to my Chrome GMail window on the computer so I rely on the phone notifications.

Sometimes I would get a phone notification of an email (both personal and work) in the GMail app hours after I had already seen the email in Chrome on the PC and had deleted it or in some cases at least definitely had read it and didn't do anything further with it.

My brother has a Galaxy S9+. This has been his and his big extended family's first smartphones of any kind. When I complained to him about the late or useless Note 10+ GMail and chat notifications, he told me he experiences the exact same thing and he assumed that was just the way Android is.

Factory resets and all disabling of background snoozing of affected apps didn't help at all.

Samsung gimped Do Not Disturb. When DnD came out with (I think it was 5.0 Lollipop), I hated it and gave up on it quickly. I don't remember at what point I started actually using it on the Pixel 1, but they definitely improved it tons. I don't recall exactly what Samsung's implementation was missing but it was very noticeable having come from the Pixel 1 and no longer being able to do some things. I think what Samsung was missing was the ultra-fine granularity of configuration I could do for each and every scheduled and common/unscheduled DnD profile as to what exceptions were allowed to get past DnD. They had some of it implemented but missing enough that it stuck out at the time.

Now that I have the P6P, I can say that the Note 10+'s night mode camera shots were really bad. I think they were worse than even the Pixel 1s'. The P6P does wonders with what little light is available, by comparison.

There may have been more but I can't recall at this time.

I did like the built-in stylus. My wife used hers tons more than I did, but it would be handy for Google Photos' magic eraser and for other things. Samsung owns patents that would keep anyone else's stylus from being quite as useful or convenient, however. If I remember correctly, between Apple and Samsung's stylus patents, everyone else's stylus just can't compare.