Asgard ships possess a system of unknown nature (since it was mentioned as a problem, but never seen in action), that prevents explosions from occurring on ship. As a result many of the tactics used against Goa'uld ships from the inside, by less advanced races (grenades, C4 and nuclear devices), would be rendered useless on Asgard ships. This technology presumably is not limited to their space fleet, and is probably fairly ubiquitous throughout Asgard facilities. (SG1: "Nemesis")