It is not just about Initiate 1, every mmr bracket is insanely hard to climb up from. This is my win-loss trend for the past 200 matches I have played (Screenshot is my profile from
statlocker).

This means that
I have a 63% win rate over my last 200 matches. How much have I climbed from this? From Arcanist 2 at the beginning to Ritualist 3 now. And I am not the only one. It takes a lot of wins to climb up from one division to the next. If you say you need 4 wins to climb one subdivision and you are in initiate 1 and then you quarter that number to account for any winstreak mmr the game may or may not give you, that is still a 66 WINSTREAK that you need to get to climb to the highest ranks. Deadlock has the most amount of ranks of any similar game paired with a very slow rank mobility system. This brings us to:
Issue 1: So many good players are stuck in the lower ranks that the best Seekers-Alchemists-Arcanists-Ritualists are consistently playing as well as Emissaries-Archons-Oracles-Phantoms which makes it impossibly hard for actual Arcanists or Ritualists to climb higher ranks.
Issue 2: When the game matchmakes these "good" low elo players in the same team, it is a huge stomp.
How many times have you been stomped by a team that is clearly much more skilled than you, but then you check the match screen and see that both teams are the same "rank"? These games are the worst, it is fun for neither the winning team and it is definetly not fun for the losing team. These are the games that make deadlock lose its playerbase.
Issue 3: When the game matchmakes these "good" low elo players against each other, the match is balanced and one of them is bound to lose, but then all this does is push the loser even farther than his actual rank, because
the game does not take performance into account for its mmr calculations, it only cares about win/loss (and even that barely affects things). This just exacerbates the situation.
Issue 4: MMR decay is insanely overtuned. If you leave the game for a while then come back, the mmr system will place you down in the lowest depths of the ladder. You will stomp every game you come across (unless you face others like you) and it will take ages for you to climb back up.
Issue 5: 4 stacks, 5 stacks, 6 stacks just turn off the matchmaker and these teams get paired with literally anyone the game can throw at them. This means that the people playing in huge parties usually gain a massive advantage (
no further words needed), along with perfect hero selection (one tricking is trivial).
TL;DR: Game has too many ranks but very low mmr mobility paired with very high mmr time decay, this makes good players stuck in lower ranks and they make situation worse for everyone including themselves. Couple that with huge party stacks and que the matchmaking complaints.