I always keep it professional, which is also why I feel like I have no choice but to share when I see pitfalls or have a better approach from past experience with seeing exactly what went wrong in similar projects. I have even dealt with people who are hard headed or say things like "fuck the customer" (really) and don't want to listen, and I just preface it with "I know but I just have to say it, it's still your decision what to do". However I've even had it come back around where the thing I warned about came true and since they were in the trusted position they were able to blame the failure on the developers! It's insane how often people will throw you under the bus just to protect their throne of lies.