I think the newness is leaving match making for people who would be in what ever tier I exist in a weird place of either being rolled or rolling most of the time.
This is a super common complaint from what I've seen, so I suspect maybe matchmaking just isn't very optimised right now. I've found that most of my matches are actually very close but there is wild imbalance between players in a match. Like you'll have one team with 3 people who go 2/12 or something and then another guy on the same team going 24/2 with a couple of other teammates somewhere in the middle.
I think another common problem for some people is that they consistently lose their lanes either because they don't understand the soul mechanics or they lack FPS mechanical skill and struggle with positioning and aim in an early 1v1. And because they're new, they don't understand the mid-game mechanics enough to realise that there are ways to catch up after a bad laning phase by playing smarter and more defensive. Instead they keep trying to push aggressively or join team fights and endlessly feed as the power gap grows larger and larger. If you are that far behind and repeatedly dying then it can feel like a game is less even than it actually is.
The invite only thing doesn't really mean anything in practice, though. I think it was more hype generation from Valve than anything else and if you check the Steam discussion page for the game it's just filled with endless threads of people giving away invites. Currently Deadlock has a 96,000 24 hour peak on Steam, putting it in the top 15 games being played on the platform and its all time peak is 171,000 from a month ago. There are definitely enough people in the pool for balancing to be possible.