It does NOT mean that the drivers are particularly broken or crashy. That would be "wip".
And FYI: there is not "NO" reduced power mode on those cards. There IS some mitigation of power consumption/heat -- enough to deal with the main problems of running a chip at full power all the time.
And again, back to the point you made about being in development... improvements to this are coming... FAST.
This driver works GREAT, and is getting EVEN BETTER.
And note: a LOT of things in this world are under development IN PERPETUITY! That means that they KEEP getting worked on until they are either no longer needed, or time itself ends. The LINUX KERNEL ITSELF is an example of this.
So don't get put off by a flag that says "in development". (Note: Actually, it says "mostly".)