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].

[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