Actually completely disagree with you. Space Hulk is an extremely balanced game if the Marines are played by someone who knows what they are doing.
Most people I play against complain how hard it is for the stealers to win, but again if played by someone who knows the tactics the stealers are effective.
Just make sure you use as much cross fire as you can in overwatch, be patient and protect your flamers. Flamers are mainly used to block map sections, killing stealers is a bonus,