Opening the book…
Most people invert this: soft to the face, sharp in the corridor. That inversion is how teams rot, because everyone eventually works out that the kindness in front of them is not the assessment behind them. Doing it the right way round means your praise is believable and your criticism is survivable.
When you have a real concern about someone's work, book the direct conversation before you mention it to anyone else. When their name comes up in a room they're not in, say the truest good thing you know about them. If you find yourself saying something in a meeting you couldn't say to their face, that's the signal to go have the conversation.
Genuine safety or misconduct concerns sometimes have to go to a third party first, and you should. Seeking advice on how to raise something is not the same as talking behind someone's back.