Receiving Behemoth support is always a moral boost to any losing team. Even when you think there is no hope for making a come back it's still a nice way to go out guns blazing. That being said I feel like some of them are just laughable. From my personal experiences I have never felt threatened when the enemy team gets a Dreadnaught. It's by far my most disliked behemoth, for the sole fact that it's soooo easy to destroy and offers very little support while it's active. Also it is currently only on 2 maps, Fao Fortress and Empire's Edge. Fao Fortress has very little cover to hide from the ship's barrages, but even then, it seems like it rarely turns the tide. In fact it usually gets sunk before it even gets in close to do any real damage.
The Armored Train on Suez and Giant's Shadow is also poorly suited. The track on Suez is on the far north edge of the map and won't allow it to capture any objectives, or even get remotely close to any for that matter. Giant's Shadow's track goes through at least 3 objectives so it serves a better purpose, but usually doesn't last long since those objectives are usually so very crowded with Assault enemies and not all the turrets can get to them. But on Suez most of the seats are useless unless the enemy decides to go after the train. The only one with a good chance of getting kills is the driver with the mortar.
The Zeppelin is my favorite behemoth so far, (the Char-2 is cool but I have yet to use it any). It offers good defense, when the pilot knows what they're doing at least, and definitely offers plenty of firepower. The pilot has control over dropping bombs that can almost one-shot a tank. Additionally there are 4 auto cannons, 2 on each side of each gondola, and 2 360 degree machine guns on to top, 1 on the nose and the other on the tail for anti-air support. (They work better than you may think against planes). If you manage to get the zeppelin and a competent pilot you can definitely turn the game around. You just gotta take out all the anti-air guns as fast as possible to limit the damage you take, then go and capture the outposts. It also makes for a good strategic spawn point if you have an open seat. To boot it counts as a kill if an enemy plane runs into it and it will kill anything that's below it if it gets destroyed. So watch out if you see it going down in flames. Even if you're in the bunkers on Monta Grappa, it will kill you if it lands on top. Then you can use its burnt remains as bonus cover. So even when it's taken down it serves as a benefit.