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.
Contested Sequel: Compared Monster Attack to Global Defense Force, the latter has two playable classes, is nearly twice as long and has at least twice the variety in enemy types. It even makes Earth Defense Force 2017 look limited in comparison.
Demonic Spiders: The spider enemies, their web attack slows you and they also jump around really erratically. Their attacks at higher levels in addition to slowing you down do large amounts of damage, which usually kills you in a heartbeat. They spam webs more and more as difficulty increases.
To a much lesser extent, Hectors. While they rarely arrive en masse in contrast to the spiders, they can soak up a lot of damage and also dish it out rather well, even knocking you away with some of their artillery weapons.
Goddamn Bats: The swarming ants and gunships especially when drop ships or a boss is present. They're not that strong individually and you can take them out with relative ease, but they almost always come at you en masse, which makes it difficult to deal with them or other threats you have.
Scrappy Weapon: The Plasma Launchers are most useless weapons in the game simply because they cannot be reloaded when the ammo runs out and there are rocket launchers (which what they're categorised in) that far more superior in terms of damage output and rate of fire.
So Bad, It's Good: The game is pretty cheap as its voice acting and graphics show. However, those parts are usually seen as hilarious and the sheer amount of fun you get from shooting hordes of every Sci-FiB-Movie monster imaginable makes up for it.
Mission 52, the second to last level of the expects you to take a massive onslaught of ants, space fighters, and some Hectors in the first wave of enemies. The second wave repeats what first one did, only this time with drop ships and those annoying red space fighters added in the mix.
A level way before that puts you in a hopeless fight against an ungodly swarm of those God-forsaken spiders — the very same cheap-ass spiders that can kill you by spamming webs that slows you down and murders you in the blink of an eye!
Even off Inferno difficulty, insect nest missions are really annoying, and on higher difficulties it can basically turn into a bloodbath in seconds.