Mech seems very map specific. Large, wide maps really favor zerg vs mech, while tighter maps can work well for the terran. The problem I find with mech is that zerg can simply drone and defend while you are building up a large enough force to push out. By the time you are able to max, zerg is able to have enough potential larvae to instantly react to your army and counter it perfectly. I feel as though TvZ revolves around constant aggression to keep the zergs economy in check.

Source: Also KR masters terran