用意周到に見えてどこかで抜けてるしな。

。

She looks well-prepared but there's always something she's overlooked.

Literal

Preparation-thorough [to-に] appearing somewhere [at-で] missing [listing-し] [sentence-final-な].

用意周到 (よういしゅうとう) is a 四字熟語 meaning 'thoroughly prepared.' ~に見えて ('appearing as, but...') sets up a contrast with reality. どこかで抜けてる ('is missing/gaps somewhere') describes an overlooked flaw. しな combines the listing し with the masculine sentence-final な, adding a tone of amused resignation — 'well, that's how it is.'