Sometimes a villain cannot do it all himself, or perhaps he simply does not wish to get his hands dirty. Whatever the reason, the bad guy may decide to employ minions. Perhaps he needs the heroes distracted to cover his escape or to keep them busy while his Master Plan is executed, or perhaps he needs them softened up so he can deliver his master blow. Whatever the reason, the hero may find him or herself up against a host of foot soldiers who can either prove to be a hinderance at least, a lethal collective opponent at worst. Below are some examples of minions as well as their respective pros and cons:
The neat aspect about zombies is their potential horror or comedic value, depending on the tone of the game and your players (I have known players who can make anything funny. As long as the player is not completely derailing your game you should just try and enjoy the experience). When presenting the zombies you should play up the atmosphere. If supernatural in origin you should go into detail regarding the horrific prospect of rotting, stinking, maggot-infested flesh, or perhaps the reanimated are people the characters might recognize. There should be a real feeling of dread and disgust when confronted with the undead.
The problem with zombies is, they are often slow. In recent years fast zombies have been portrayed in various films (and in a steampunk novel I read called Boneshaker) that they can be pretty quick on their feet, and they are fine so it is up to you as to what kind you want, so this might not be a disadvantage. The only other drawback I see with the undead as your foot soldiers is they lack brains and so they can be often out-witted.
And that is the big problem with robots. They are boring. Fighting robots can be a very dull experience because in the end they are mindless, soulless creations who when destroyed offer no consequences for the players to worry about. It is why the climactic fight in Iron Man II was so inferior to the one from the first Iron Man film, why the second season of Legion of Super Heroes was so inferior to the first season, why the three part season finale of Avengers which was recently aired was hit or miss. And especially one of the reasons why the second Star Wars trilogy sucked. Robots can be sleep inducing.
That is not to say robots do not have their place. Robots used sparingly can be all right, especially if they are interesting. In a Dungeons & Dragons or Wild West campaign, for example, fighting steampunk clockwork minions would be an interesting change of pace. I just feel it is vitally important that GMs do not rely overly much on robots as plot devices.
Do you know who modern ninja are? Green Berets. Navy SEAL teams. Russian Spetsnaz, British SAS, pretty much any military unit whose members are given training far beyond the norm and are pushed well beyond the limits normal soldiers can endure, are outfitted with special weapons and gear and are given special high risk assignments. That to me is what ninjas were, special forces units with extraordinary training and equipment, given tasks conventional forces were unable to perform. If you have a ninja idea in mind you might want to go the route of using modern special forces soldiers instead, perhaps a made up unit. Just don't call them "Ninja Force" or the like. In fact, if you go this route giving the special forces team in question code names like GI Joe characters might be a neat idea. There is nothing wrong with giving minions a little depth and flavor.
Gangsters have the benefit of being potentially real people with potentially real problems. Why do people become gangsters? Greed? A need to support a drug habit? A need to support a family? What are the underlying reasons for them being gangsters in the first place? Perhaps that roving band of raiders are in reality displaced soldiers who have turned to minionhood for the big bad because their own kingdom has been overrun by another big bad? The important thing here is that gangsters are people and as such hold in them the potential for plot hooks.
Soldiers can offer a possible political aspect to your game. What if your super heroes have to enter a foreign nation and are confronted with armed soldiers? If they fight them they could cause an international incident. Like gangsters soldiers are people, too, and even if they serve a corrupt regime (i.e. Saddam Hussein's Republican Guard) slaughtering them wholesale may not be the most moral thing to do.
Natives provide the same moral quandaries some other minions provide, and that is how moral is it to attack peoples who have a legitimate beef against the heroes? Or is killing them the proper thing to do if it creates more problems than it solves? I would say the only drawback to the use of natives is the potential racial factor. Be very careful not to fall into racial stereotypes, as it may offend some players.
Well, okay, honestly if kobolds were not meant to be slaughtered in a genocidal frenzy they should not have been born neutral evil.
In closing, when using minions think about what you as the GM wish to accomplish in terms of plot and tone. Your minions can be mere cannon fodder or a plot element in themselves, can be employed to generate humor or horror.