The problem has been dicussed before in other threads. Sum of points gained by the winning team is often larger than the sum of points lost by the losing team. Therefore new points are created out of thin air, which violates the rules of ELO-systems. The effect is that the average rises, so inflation of the worth of ELO points and the top will get higher and higher. This does not have to affect the MM-system negatively for active players, if it’s well designed. But if a player doesn’t play for a long time and comes back, then his ELO will be too low for his skill level due to the inflation.
I already got suspicious of this fault early on: on my first 1v1 game I started with 1000 ELO, but on my first team game I started with an ELO of 1318. So likely the average at that time was 1318 and the system gives a new player in the ladder the current average ELO, which is a correct mechanism, except that the average ELO should stay on 1000.