as smith said, its not that the emmisary is strong, its that the mavor is weak.
http://faforever.com/faf/unitsDB/unit.php?bp=UAB2302,URB2302,UEB2302,XSB2302in comparison with other t3 arty it is about the same, and i think cybran arty might be even better is it fires twice as fast and has more aoe, so it can hit a ton of shields in one shot, and microing shields vs it is even harder.
comparing mavor to its counterparts we see its useless.
http://faforever.com/faf/unitsDB/unit.php?bp=XAB2307,UEB2302,UEB2401,URL0401salvation costs 10% less and has a LOT more aoe and 40% more dps
scathis has same dps and more aoe, for only 40% of the price, and less range and less precision is not worth 140k mass
uefs own t3 artillery can be built to have a bit more dps, and it will work much earlier as you don't have to buy all 3 at the same time. also you get more total adjacency bonus as there are 3 of them so you can put more t3 pgens next to them
the counter to t3 arty is to fill your base with shields. one t3 shield uses 500 energy/s and can stop an emmisary shell if you build 10-15 of them they should stop it for a long time,
and still cost less than the arty did
http://faforever.com/faf/unitsDB/unit.php?bp=URB4207,UAB4301,UEB4301,XSB4301