Fusion Thrusters are incredibly useful on most ships, especially battleships, destroyers and carriers because they lack turning speed and require forward speed to catch up to or run away from enemy fleets.
Equipping this thruster on frigates, cruisers and cutters allows them to quickly switch between running and attacking, giving opponents a nasty surprise.
Fusion Thrusters decrease strafe speed, although the penalty is the exact same as Rear Thrusters III-V. They are unsuitable for long-range frigates because they rely on their strafing ability to kite enemies.
The decreased strafe speed also damages your ability to dodge incoming projectiles, making them unpopular on sniper fleets. In addition, shield-switching becomes more difficult because you would have to turn away from the enemy in order to cycle your ships.