I think it's important to note that sometimes a vocal minority will make it seem much worse than it is because the majority aren't bothered enough to give their opinion on the player being fairly good.
Smalling is the best example of this. Reading this forum at times it felt like everyone disliked him and wanted him out, but every time a vote or poll came up it was always Smalling +1. Then when the latter group are more vocal about "Why are we selling him" or "I wish we'd given Smalling+Maguire a chance", the former group start going on about them thinking that because he's no longer here and the grass always being greener. No, they always thought that but didn't feel strongly enough to post it, and it was the first group who were basically in an echo chamber that made them think everyone agreed with them.
Overall it tends to be people with short memories. Martial is copping it at the moment because he's started this season poorly, despite the fact that he was probably our best player over the whole of last season and we largely got into the CL spots because of his form after lockdown. Rashford likewise in the past, although I actually think he's getting off very lightly at the moment because people are focusing on Martial while Rashford's barely been any better.
Generally it's people going overboard with their criticism, and pretty much every player has been on the end of that. People going on about them barely being a premier league player and some such nonsense. Unfortunately with the media and social media we live with today it all encourages over-the-top point of views rather than weighing up the good and the bad fairly. It's how you get clicks and reactions.