Asaf has composed a FAQ on Flagging, but I have a rather specialized concern that I did not see discussed yet.
I occasionally see Comments that, doubtless with best intentions (for Questions that are not on-topic here), recommend another SE site which the commenting User imagines would be better.
When said User is not an active participant on the other site (but I am), I am tempted to flag for moderator attention/removal those suggestions I know to be inapt. The impulse to provide a better alternative is generous, but most Communities have a highly honed set of criteria for what they consider on-topic, and outsiders and casual (non-posting) members will likely not appreciate those boundaries.
I think it is better (for those who don't actively participate on the other site) to recommend looking at another site's Questions for similar items, which avoids the immediate cross-posting/self-migration of items that may not be all that well phrased or thought out to begin with. In practice I may do such a search for them, to give some tags or links to possibly related or duplicate Questions.
If a Question is closed or on-hold here for being off-topic, I don't mind posting a counter-Comment explaining why the other SE site isn't a good alternative, but this is more for the benefit of OP (rather than corrective action vis-a-vis the suggesting Commenter).
So, is it reasonable to flag Comments with really bad suggestions for other sites?