Short answer: No more than 420nits on X3 and X3 pro units I have had experience with.
Long answer: Before X3 NFC I had Oppo A5 2020 with Maximum 480 nits advertised brightness.
Now I have a Poco X3 and my brother has a Poco M3
If I compare my X3 with Poco M3 (400 nits advertised) and Oppo A5 2020 (480nits max advertised) and consider that other phones perform at their advertised brightness.
I perceive that X3's maximum brightness is no more than 420 nits indoors and outdoors both. There's a shift in screen contrast / gamma value which ruins picture quality but no 550-630 nits boost I read about in reviews.
I think Xiaomi uses multiple grades of hardware components for same device.
Review units and the units sold in EU and Wealthy Middle Eastern countries like KSA, UAE etc are made with A Grade components
While those sold in India, Pakistan, Philippines, Indonesia etc are made with B grade components. Although some A quality units make their way to these countries too but those are rare. (I guess varies by available stock and warranty providers e.g. there are 4+ distributors/ warranty providers) of Xiaomi phones in Pakistan.