Been fascinated with the theoretical power ratings of connectors - it is often much higher than you would expect, especially for diminutive connectors.
Here, for example, is a power rating graph for an inline female BNC connector:
If I’m reading that correctly the power rating for 2m and longer wavelengths rises from 3 kW.
I tried to standardize on N-Type connectors in the early days of my radio hobby, having read of the various inadequacies of the PL259/SO239 combination as the wavelength shortens. However, they are heavy for SOTA, BNC appealed much more for this application, but I did also note in that data sheet that the BNC durability is only rated for 500 matings, which isn’t a lot for a prolific SOTA activator.
Mating cycles is one of the differentiators between mil-spec and consumer connectors. When my job was to make the stuff to ensure HM’s Navy could put a selection of big ones into Moscow etc. there was the specified rated number of mating cycles and the allowed number. Our allowed number was very much less than what was specified. Exceed that and the connectors had to be replaced which limited people’s inclination to plug and swap gear about.
The power is limited by the dielectric strength (for BNC that is 500V) and current handling. There are some high voltage versions available and they look different to normal BNC connectors. Then you have to consider the VSWR as that will limited the power as it rises. The power handling drops as the frequency rises as the current moves more and more into the skin of the centre pin i.e. there’s less copper to handle the current.
That chart seems very optimistic IMHO. 350W in a BNC at 1GHz it what I seem to recall. 1500W @ 1GHz in a TNC, 1800W @ 1GHz in an N and 4000W @ 1GHz in a 7-16 DIN.
This is, of course, using proper coax and connectors and not cheap tat from China and eBay.
The lack of consistent VSWR for them is not really an issue at amateur powers. The biggest issue is the way the coax is connected and the fact that cruddy ones are unbelievable cruddy. They’re heavy and naff so if you want a proper connector, BNC and the less popular TNC are much better choices. For real QRP at HF, a phono plug is just as good and very light.
You could often pickup very high quality TNC connectors at rallies (ha, remember rallies!).
There is no best connector, just the one that does the job you need for a price your wallet is happy with.
If I were considering putting 3 kW through a pair of BNCs I think I’d being worrying about passive intermodulation too.