The big feature of Bird Wattmeters is not absolute accuracy, but rather repeatability.
In commercial use, I have mainly seen them in use as in-line indicators in installations where the original tests would have been done with much more accurate (& expensive) equipment.
The Bird Wattmeter was then placed permanently in line, & its readings recorded.
If it is reading high or low by a particular amount, it is very likely to still have the very same error in 20/30 years' time.
A Tech can visit a site, quickly check the reading on the "Bird", & if it hasn't changed, be reasonably confident that there is nothing amiss.
if it reads differently, it is time to "bring out the big guns".
Ultimately, some of these Wattmeters reach the Amateur Radio market, where they are prized because of repeatability.
Although some of the similar devices built for that market, & indeed, the CB one, offer similar accuracy, they are "built down to a price point" & due to poor mechanical design & bad soldering may fail in a relatively few years.
Of course, you know all this, but no matter how many times you tell it, Chat GPT will never know, as it treats every question as if it was the first time it was asked, & does not learn from interactions.