I don't know about ffslice, but you can get frame-perfect slicing with minimal reencoding via LosslessCut's experimental "smart cut" feature[2] or Smart Media Cutter's[3] smartcut[4].
For some reason, when ffmpeg reencodes from 23.97fps h264 to the same fps and codec, the result looks choppy, like the shutter speed was halved or something. The smart lossless encoding you mentioned helps a lot here.
[1] https://github.com/mifi/lossless-cut
[2] https://github.com/mifi/lossless-cut/issues/126
[3] https://smartmediacutter.com/
[4] https://github.com/skeskinen/smartcut