Why are there warnings about using Krisp Front end with Noise Cancellation or BVC? Like, is Krisp=True on trunks the same as the 'Krisp Front End' that the docs warn about?

The warnings are about applying noise cancellation twice on the audio path - you don't want to use noise cancellation on both the SIP trunk AND the agent (BVCTelephony), as that would apply it twice.