Are you talking about client devices or APs?

If the latter, you can’t have 802.11r running across different vendor’s APs because a central controller has to coordinate things like WPA pre-caching and exact timing of the roam event, so if you were to set up a Juniper AP with the same SSID & WPA as a nearby Ruckus AP they would not be able to centrally coordinate roaming hand-offs between each other.

It helps to remember roaming in the 802.11 standard without 802.11r is a client-side decision (unlike cellular); 802.11r essentially uses a few dirty tricks (IMO) to make it a network-side decision and remain backwards compatible. Unfortunately not all clients handle that behavior well even if they claim to be 802.11r compliant (and some vendors don’t even bother despite living in 2025 - looking at you Ring Doorbell!)

I’ve also seen some vendors implement their own tricks into the mix above and beyond the standard, with varying degrees of success.

Because standards don’t cover everything and each vendor will implement the standard how they see it should work.
Which means when a Cisco AP is doing its thing, a Juniper one will be doing it slightly differently.

Welcome to the vendors vs. standards world.

Depends on vendor and standard, interoperability is challenging, sometimes impossible.