I did the same with a SanDisk 400GB card. Works too.

The limit could be the SDXC spec: 2TB.
I guess at the time the N900 was designed nobody believed that it would ever encounter a MicroSD which maxes out the spec
(It wasn't even designed to handle SDXC)

We still have to wait for 2TB, but 1.5TB is already on the market.

Also 2TB is

But as the SDUC from 2019 is compatible with the standard "High speed" pins, the limit should be even higher. You just can not use the fancy ultra high speed modes.