This is really the biggest problem with Overwatch Competitive. You never know who you'll be matched up with (unless you're in a group) and since Overwatch is a team based game, being matched with bad players almost always ensures your ranking going down, which for some can be frustrating,
Wouldnt really say it's a problem per se, like the implication here is that the player is some shining star held back by their ppl. I think the key thing here to consider is the ease in which a player can carry a game.
For example, ones shitty environment at a certain rank should be true across the board at that particular rank (not saying that all players at a rank are equal, but that if we associate a rank w/ shit there should be a high chance that there is shit on the other team). In mobas, which are heavy in terms of player responsibility and team play, there is a certain extent in which you can carry a game (that is, you can do your job and more than counterparts on the other team). The ability to carry in a shooter plays an even bigger role, like if you're the shit it really should make a key difference in the game. The ability to carry a game where you are better than your rank is pretty high, not to mention the ability to communicate, switch roles, etc.
The ability to rank up and down kind of hinges on this. Naturally ppl will focus on games that were difficult for them to carry rather than games where they were carried tho
Importantly, personal performance is included in the calculation w/r/t points lost and gained, hard to really talk about it since it isnt transparent, but i went 8-2 in placements and got 57 while a friend went 7-3 and got 61, and it felt totally deserved.