An ideal editor would be one that only re-encodes between GOP boundaries and leaves the remainder of the stream untouched. Apparently VideoReDo TVSuite H.264 can do this, but I've never used it.
With AviSynth, I have had problems with reading h.264, specifically with the frame accuracy of source filters. FFMPEGSource2 gives me dupe frames on some streams, DGAVDec is very slow, more accurate but still serves the wrong frames sometimes (and is no longer supported by its author).