Heavy's, why yes there are good.
They are in many times for me they are the key to winning firefights. A heavy is meant to be up in the front line taking up the fire. If you plan like most people use a certain gun, "Carb" Do not bother being a heavy. Try everything out and see what you like, everyone has a favourite thing. I have a heavy soldier and I love being up in the front lines.
Note: They are not that slow they keep up with the team rather well.
Enjoy the fight for the Ark