What is a typical upper frequency limit for low SWR operation of 50-ohm BNC connectors?
Why BNC is a small bayonet-lock coaxial connector with a controlled 50-ohm geometry, so it holds a good match well up into the microwave region. The pool's figure for low SWR operation of the 50-ohm BNC is about 4 GHz. Above that, the connector's internal dimensions start to allow moded behavior and the match degrades, so precision types like SMA or N are used instead.
Watch out The 500 MHz figure is the common practical ceiling quoted for the UHF (PL-259/SO-239) connector, which is not a constant-impedance design; 40 GHz belongs to tiny precision connectors such as 2.92 mm.
BNC = 4 GHz, UHF/PL-259 = a few hundred MHz. Small bayonet, big bandwidth.