Any hidden alt is shady in my book and warrants some type of feedback for awareness, negative or neutral may depend on the situation. My issue has been how much proof is required to reasonably match someone as an alt. I try to conform myself to a community standard that I thought was in play but others seem to cross that line unchallenged, so I'm not sure why I'm holding back. I do not want to dampen any progress that may be getting made but I see a lot of these unusual merit links as hunches because they aren't really based on any hard proof, unless there are other links between the accounts that can be found in addition to the unusual merit activity.
I've been led to believe that it is not safe to conclude someone is an alt based solely on potential shared wallets... which I would think is a more decent link than sending merits to another account, but again, my impression is that this isn't enough to reasonably link accounts. That being said, it is hard for me to ever conclude that it would be ok to accept someone as an alt because they sent X amount of merits to someone... even though my gut feeling says otherwise in some of the cases.
It would probably be a bad idea to use received merit as a basis to corroborate linked accounts because it is fairly trivial to make someone look like a scammer if this is used.
I also believe these people have a high percentage of false positives in leaving negative trust against the innocent.
Any trust that you leave (and trust left by those directly on your trust list) is ultimately a reflection on your reputation. Leaving negative trust against an innocent person can have negative consequences for said person (many of these consequences may not ever be apparent to you), and is especially troubling if negative trust is left with flimsy evidence and/or when reasonable explanations (explaining innocence) cannot be ruled out.