I would assume good methods to nerf bandits would be range, accuracy and damage, as well as keeping them to specific areas.
Especially accuracy and damage could be tuned to be very bad or even guaranteed misses the first few shots and then continually improving. That way you would always have time to just go behind some corner and give yourself time to orient.
Also bandits could very well only occur in bandit camps (and then in the center of cities only) and possibly as wandering hordes in late game. After all bandits may be end-game opponents only, at least that was the plan stated by TFP a long time ago.
Modders often tune their mods for advanced players and they expect those players to know how to handle them even at low level. TFP has to think about novice players, IMHO it is very likely that experienced players will be a bit dissappointed by the difficulty bandits actually provide. Though if they are relegated to end game you should expect difficulty on par or even higher than the demolisher, but only if you play that far.
EDIT: Just a wild guess, but maybe they hope to make them really dangerous only if they act as a group, i.e. that a single bandit is easy to kill because you can easily exploit its AI, but in a group you would have a few bandits pinning you down with low damage shots while two others try to narrow the distance to get in shotgun range. So in mid-game you would see single bandits, but the real danger would come with groups of bandits.
