These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Bill shows no mercy or sympathy for his victims and sticks to his principles. His principles being to relentlessly and ruthlessly kill a large amount of people to decrease the strain of overpopulation (that isn't as big of an issue where he is, in current day), frame his only real friend for the entire ordeal, and make off with thousands of dollars to fund future rampages. The guy doesn't even pay for his sandwich!
Special Effects Failure: Generally averted, but the guns Bill is carrying have a tendency to change between shots - a gun that's obviously a MAC-10 in one scene has magically morphed into a Micro-Uzi the next scene and so on.
Maybe he has multiple guns?
There actually are no gun-switching goofs. One of the weapons Bill wields, a Ruger MP9, looks like a hybrid between a MAC-10 and Micro-Uzi. See for yourself, here.
Base Breaker: Rhett from Total Destruction. Some fans like him because he can jump high, is quite agile and speedy, and just looks pretty cool like several of the monsters. Others think he's a Replacement Scrappy for Curtis from Through Time and Universal Tour.
World Tour has the small robots. They look like Goddamned Bats, but they take quite a few hits to destroy, and if you ignore them for too long, they'll easily chew through your health with their machine gun attack. It doesn't help that they launch their flamethrower attack at you the second after you hit them, which is very hard to dodge.
The same game also has Super Mechas. They're gigantic walking robots that will slowly approach you and promptly bash you in the face. They're hard to kill, it's hard to dodge their attacks, and even if you do destroy one, they devolve into the regular sized robot mentioned above. It is however, possible to deal with them easily be using repeated jump kicks, and their slow speed means they're easy to avoid.
TANKS. They go down after three hits, but a single shot from a tank will knock you down or knock you off a building. And they rarely ever stop firing; chances are, the second you get up, you'll get knocked right back down...
Bombers are the flying version of tanks. They go down after one punch/kick, but they fly really fast and their bombs will instantly knock you off a building.
Flamer Dudes from World Tour. They're soldiers flying on jetpacks and carrying a huge flamethrower. They die after only three hits, but their attacks tend to set the monsters on fire, which leads to them falling off a building.
Super Flyers from World Tour are a cross between this and Demonic Spiders. They're weak, but their laser can drain your health if you ignore them for too long.