I probably spent 20 hours this week trying to get a not-that-obscure piece of software that explicitly states "Debian 12" support on their website working on a freshly-installed (as in - install Debian 12, update system, install software); and it still just doesn't work. After hammering my way through 4 different errors, and hopelessly out of date (and wrong) documentation, I finally got them to issue a refund this morning.
"Linux Compatibility" often means "someone got it working once 6 years ago, on an extremely out of date version of the OS, but its probably still fine".
This is why you do an internet search instead of just looking at what the vendor says, and/or look at what the vendor actually ships for support. If you see something like a binary blob, run away. If you see stuff shipped upstream, go ahead.
As a counterpoint to your experience, I've had great luck buying laptops off of Canonical's approved hardware list.
As a corollary, I've had worse luck buying computers that come with Linux pre-installed, only to discover that the pre-installed distribution is very specifically and carefully configured and includes proprietary bits and it's impossible to install any other Linux on the device or upgrade it.
"Linux Compatibility" often means "someone got it working once 6 years ago, on an extremely out of date version of the OS, but its probably still fine".