After around 30 hours of play time I feel like you could compare this game to a ski course. Going down the easy normal or hard ski courses will be fun for the first few times, but eventually it will start to feel like the same thing over and over again. The missions are always very similar to one another with minor and major changes to shake up the experience. Going on the lower difficulties will sometimes offer a challenge though, basically they lodged large rocks in the ski hills that you have to overcome.
These missions are sometimes extremely difficult and will be almost incompletable without the help of another player in up to 4-player coop. When you play on hardest or inferno however, they will lodge a whole lot more enormous rocks in the hill and it will feel like you aren't making any progress, but I feel like this is where the game shines the most on an enjoyment level. You will have to plan out each action you or your friends take, and learn from each failure while trying different weapons to complete different challenges. You will have to all work together and coordinate yourselves to overcome some of the huge boulders that will impede your progress. Sadly, if these take too long it will feel like you won't be making any progress, but if you are looking to not mindlessly shoot bugs and actually think and plan out your actions, then the harder difficulties are for you.
Not to say that the lower difficulties aren't fun and can't be hard because they will kick you in the teeth every now and then, and I think if you like making constant progress and trying newer and cooler weapons, the lower difficulties are for you. It can feel like a crawl when you try very hard to complete a mission on hardest, only to find out you still have a 1% completion rate. Overall the game will make you very angry but also will bring you some of the greatest times I've ever had in a video game. The difficulty spikes are rediculous and most of the time after completing that hard mission you will go back to it being incredibly easy, but it is still fun nonetheless. Fully worth the purchase no matter what your play style, just get ready for a lot of shooting bugs and skiing.