[Xiaomi Mi A3 Questions & Answers] Catlish:
Long post, you've been warned.

Well. I haven't installed roms, nor have a n8, but I can tell you my experience of 3 months with the thing running stock.

The thing with the fingerprint scanner is that it's a little tricky, I think it's over sensitive. If your fingers get wet or moist (sweaty), and the skin "expands" then it's gonna fail to pick your fingerprint. If you press your finger considerably harder or softer than what you did when registering the fingerprint, then it's not gonna recognize it either.

But it's a matter of getting used to, I mostly fixed it by registering my fingerprint using the same pressure that I use when normally handling the phone; also, registering the thumb I use the most in 3 of the four "slots" with different "moisture conditions" really helped. The other two slots I use for the other thumb and an index finger, which comes in handy sometimes.

Actually, lately I've been trying with registering only the tip of my fingerprint, and I've had good results. The tip of your fingers won't expand as much when wet, having sweaty hands, etc. So, using just one slot for that thumb works like a charm, maybe register two slots and you're gold.

Other problem with the stock fingerprint is that the animation robs around 1/2 second of time when unlocking, here in the forums is a tread on removing the animation (no root required) and the difference is noticeable, almost twice as fast.

Other thing that affects the speed is the display. If you have the option to have always on show the fingerprint icon (and/or the AOD on A10), the time for waking up the sensor and reading the fingerprint goes down around 1/4 second vs having the screen fully off. I have this feature off, because I fear having the fingerprint icon always on, might cause screen burning in the AMOLED panel, and I plan on keeping this phone for some years.

Also, the Android 10 update made an improvement in speed and accuracy of the sensor I could immediately notice it after the update.

Having said all that:

If you configure the fingerprint the way the phone likes it, and have the speed tweaks I mentioned above, the successful read rate is around 95% and the sensor speed is nearly the same as a capacitive sensor. Arround 0.33 seconds for the a3 vs an average capacitative sensor's 0.25 seconds, which is a barely noticeable difference, really.

In the way I have my phone configured, I would say the average time I spend unlocking my phone is around 1/2 a second. (screen totally off, animation removed)

Now, the redmi's sensor:

There's no discussion. That capacitive fingerprint scanner will ALWAYS be faster and more accurate than this one. But, honestly that additional 1/4th of a second is something I can live with easily.

About the screen: I'm not gonna lie, you can see the difference in sharpness because of the difference in screen resolution of the note 8 and the mi a3. But really, I don't consider it a problem as many youtubers do. I can read the small text just fine (which is what I consider most important) and because it actually affects battery life in a positive way, A LOT.

The reported average SOT of the RN8 is of 7:30 hours, I would say the average user on a mi A3 will get ~8:30 hours of SOT. Because of the lower screen resolution and blah blah blah.

(my average sot is 10 hours btw, that includes playing games and YouTube for 1+hours each. And in days with no games it comes to arround 12h with 10% left. Notice that above I mentioned the "average user". I don't feel like comparing my usage because it would be unfair; I am not an "average" user, I actually care a lot for my battery life. Screen brightness is almost always low, black amoled backgrounds, dark mode, little background apps, etc,. But. It pays off, max SOT I've had is 15H and no user on a note 8 can achieve that, I'm sure.)

Now, finally. This phone or the RN8?

Well, it's simple. If you don't care about the little camera quality drop or the Android one updates and want a phone that has no hassles, doesn't needs tweaking, and works flawlessly (fingerprint, software) out of the box, then go for the redmi.

And if you want this phone, because you know you can take advantage of its amoled display, and the subsequent battery life improvements; and are willing to "tweak" it, and to iron some software bugs yourself, well, don't look back and buy it. Most of the stock android bugs are really minor at this point, might get officially fixed soon. And if not, most can be fixed with no root.

For me, for the price I paid. I have no regrets of buying it. I really couldn't have bought anything better for that much. Knowing how to get the best aspects of the amoled panel has proven invaluable because of the battery life and since the fingerprint works like 95% of the time, I say it's OK, and don't really feel like complaining.

Plus, I'm one of those weird ones that prefers simple stock android.

Sorry for. The long post, but I feel this is what I wish someone would have told me when I was looking to buy this phone. I did a lot of fruitless research too before buying and those YouTube videos didn't say anything about the crude reality, it's clear that they don't test the phones for more than an hour.