Goblins are whatever the DM wants them to be, and without having to house rule things, goblins with classes are just as deadly as most other races with classes. Sure, the goblin mooks that simply guard the warren entrance may be a pushover, but the elite tunnel-crawlers, who draw enemies into quarters too small for them and then strike, are another matter. And what about the worg-riders, trained to fight from the saddle of their lupine mounts, or the night fangs, the goblin rogues and rangers that ambush elves in the wild and sneak into elven villages? Or the priests of Maglubiyet, empowered by their god to greater prowess? Or the war-chief and his elite guard, skilled enough to take down giants through teamwork and tactics?
Goblins can be whatever the DM is creative enough to make them.