Ok, so Birds aren't the ultimate, but their accuracy is acceptable in many applications, they are rugged, the inserts (including those from Coaxial Dynamics) are interchangeable between different units, & their readings are pretty much repeatable over time,---the downside is, because they are used a lot in industry (often in fixed positions to facilitate a quick check of "forward & reverse power"), they are costly to buy.
When I used to travel around the State, doing routine checks & fault callouts on TV & Radio Tx sites, we used a HP410C, MODEL 1I036A AC PROBE, a MODEL 1I042A PROBE "T" CONNECTOR, & a known accurate test load, (I don't mean to yell, that is just as they appear in the 410C manual) to spec check the output power of the smaller Tx, up to around 2kW,------we wouldn't have used a Bird there!
Big Tx usually had a water, or sometimes oil, cooled load .
You read off the temperature change of the coolant, & calculated the power-----good old 19th Century technology!!
All the latter stuff isn't anything you would leave connected up, so simple, rugged devices like the Bird have a definite place.
For Amateur Radio fans, who over time, may have had several " ham" VSWR meters of varying accuracy, but all with limited life, the Bird is, indeed a revelation.