You think that's bs about him struggling? Man did you see any of our games at the time?
When Mou came and bought Zlatan, Martial was sad because he wasn't gonna play as a striker and Zlatan got no9. Then Mou's 2nd season he couldn't (didn't want?) compete with THAT Alexis. And finally this year he was abysmal and he couldn't compete with the 34 year old Cavani. What more do you want?
Overall he had 2 great seasons (his first one and last year) and 4 mediocre to shit.
That's not completely true though. Mourinho quite obviously didn't like him yet Martial forced his way into the starting line-up two different times through out-performing his competitors.
15/16 - Good season
16/17 - Poor season. Moved to a new fulltime position on the left, playing under a new manager who obviously didn't like him, stripped of his shirt number, and having a very messy break-up with his ex who was making it public. All as a 20yo kid. Greenwood just had quite a similar season (at least until the final month or two when he finally started kicking into form) and he 'only' had to deal with the issues with England at the start of the season.
17/18 - Was comfortably outperforming Rashford for the position on the left, but Mourinho kept favouring Rashford for far too long. Eventually even Jose had to install Martial as first choice. Despite being our best performing attacker at the time, once Sanchez came in he got moved over to the right (where he's never been able to play) and never got a run of games again.
18/19 - Started the season in and out of the team again, had a few great games and a few stinkers (which was still performing better than Sanchez). Finally got a proper run when Jose was about to be fired and (along with Pogba) pretty much single-handedly kept him in the job for a couple of extra months. Second half of the season he was in and out of the team through injury and illness.
19/20 - Great season.
20/21 - Dreadful season.
So I think it's unfair to claim he was simply struggling with the competition of Zlatan and Sanchez. He was having to deal with a hell of a lot for a young player in the first case, and in the second he wasn't given the chance to compete for way too long. At other times he happily competed with Rashford and Sanchez and beat them both. This season with Cavani is really the first time where it's accurate but the hope is that it was a one-off and may or may not have anything to do with Cavani.
Overall it was two good seasons, two seasons where he was good when he actually got a run of games but didn't get to show whether he could maintain it (through no real fault of his own either time), and two poor seasons. So while he's definitely been inconsistent it hasn't been as bad as you made out.