@rschwieb In my opinion answers that are obviously unjustifiable do not warrant a comment. For example, try h(x) = −1 and g(x) = x, for any x∈R. Since this demonstrates a basic conceptual failure regarding real exponentiation, it doesn't warrant a comment, because such users ought not to be posting so-called 'answers' in my view (and there are too many such users on Math SE to count).
@user21820 I think the (unfortunate) upvotes suggest it would be a good idea to say just that. “This obviously doesn’t work: look at .. for example.” If it had 0 votes I’d probably say nothing too.
@user21820 The total nonsense is up for deletion. @rschwieb: Thanks for commenting there, but in this case there is no point because this user has a track record of posting such no-quality stuff.
@user21820 ok cool. I just think it makes your downvote more effective, especially since it was invisible at the time :). It also saves everyone else from having to find the problem, and greases the rails for them to justify a downvote.