The matchmaker sucks. Many matches are loses before they begin due to the ridiculous comps it generates. 4 Warriors and a Guardian vs a varied comp? Lose. 3 Thieves? Lose. 5 Elementalists? LOSE. Over half my placement matches were over before the match started (ignoring the 3 4v5s I was on the losing side of).
If the matchmaker cannot make good team compositions, then the matches it makes will not be competitive. The simplest solution is to just, finally, after years of people begging you to, TURN OFF MULTIPLE PICKING. Only allow 1 of each class. It won’t solve everything, since there will still be bad combos, but at least it will solve stacking.