The Jem'Hadar have been bred to be soldiers -- they don't need to eat, sleep, or fuck. They don't have any hobbies or interests outside of combat, so they practice fighting all the time and have no qualms with killing. They are huge, strong, and have armored bodies. They should be able to absolutely destroy any human in hand-to-hand combat, especially someone in Starfleet whose hand-to-hand combat largely consists of wimpy holodeck training with safeties engaged. They are further from normal humans than Captain America and he can take out a room full of people who have the same amount of training.

I think the Jem'Hadar are designed to be bad at fighting, whether it's to prevent them from turning on the Founders or maybe the Founders just think it's funny. It keeps them from living too long so that they don't become a threat, since more Jem'Hadar can just be grown to replace any fallen ones. They are competent when it comes to flying ships, which is probably more important when it comes to defeating enemies.