I think it largely depends on the situation and personal judgment. It is not a good idea to always wait until a scam happens to warn the community. If there are obvious red flags and valid reasons,
You personal judgment (if wrong) should not damage others reputation.
How would you assure I will not scam at some time? It's not good either to stay suspicious always.
Where is the obvious red flag for starmyc and the same red flag for nemgun?
Why would I suspect starmyc and not suspect JollyGood?
If nemgun were to tag then all ICO that failed should be tagged too. Scam and fail has too different meanings. Scam is when you take from others in bad faith, a fraud.