Before さ and し it is weakened or incomplete. When ひ or ふ come after ん in a word (or compound word), they are transformed to the non-fricatives ぴ and ぷ . E.g.
- [反比例]{はん・ぴ・れい}
- [染筆]{せんぴつ}
- [選評]{せんぴょう}
- [認否]{にん・ぴ}
which confirms you implicit assertion that ん followed by a fricative is somewhat troublesome.