Don't expect great HF performance in BNC terminators. In its fairly common use - to make a 1 meg parallel some tens of pF scope input look kind of like 50 ohms - you'll always have the unavoidable capacitance in shunt. It will be be 50 R just fine at low frequency, but it's not at all equivalent to a true 50 R resistive RF/microwave system. No matter how good the terminator is, the problem is in the scope's native high-Z input structure.
Ed
