BNC is rated to 400 MHz. I have used TNC up to 1 GHZ on some MIL-STD hardware I designed. I don't remember its specified frequency rating.
I think the RF construction (as against the physical construction) of the two types (N or BNC) is not very different, at least when connecting either one to a coax like
LMR 240. For LMR 400, I would not like a conversion to BNC. If you need LMR 400, for most purposes you should stay with type N. BTW: BNC connectors definitely
leak RF at higher VHF frequencies. If you are concerned about RF leakage from a smaller coax coax connector, you might consider TNC. It's not very popular, it seems,
but it is a screw-type version of BNC, and is more RF-tight.  If you have sensitive receivers and need to keep rf out, don't use BNC. This is a job for TNC or SMA.

(I used to use TNC and coax for medium voltage DC connections in my ham shack using 50 Ohm coax, back when stations were modular--say 1965.)

As a working RF engineer, in some 1 GHz RF designs, I have used TNC at around a KW peak rf power, with semi-rigid coax. Proved out and tested in Mil-Spec test routines.

Join my crusade against UHF connectors! Let's get them out of ham radio! They're over 75 years old, designed when the best RF engineers were just trying to get
their arms around COAX CABLE. I don't think the MIT papers were even out yet.