Make no mistake, this certainly isn't a terrible game, but I've played better, and it's got a lot to do with how it's only difficult due to poor design choices.
Simply put, those new ships and upgrades you can buy and upgrade over the course of the game? Not optional. Not in the slightest. You're going to notice early on that your performance is kind of hamstrung by the fact that ALL of your ships have an absolutely abysmal reload rate, and the only way you're going to remotely keep up with the increasing onslaughts that the enemies throw at you is to upgrade your clip sizes and reload rates to the max as early on as you can - and when I say reload rate, I don't mean the reload rates for your main gun, which you cannot upgrade the reload rate of by any means.
Well, that's not so bad, you may say, doesn't that add to the challenge of the game?
It does. Right up until you start realizing that you are literally having too many enemies and missiles thrown at you to shoot down unless you buy a lot of expensive single-use powerups, and I mean expensive as in "you will have to grind entire early-to-mid-game missions at least once to buy a single pack of the only truly useful one", and that's not even factoring what it's going to take to get the late-game ships and upgrades that, again, you're absolutely going to NEED in order to stand a chance.
Basically, you can't beat this game without grinding like crazy, and that's not challenging. It's a timesink and nothing more. It's really too bad, because there's really not much wrong with the game itself - it's simple, good, lead-the-target fun that allows you the chance to negate all damage if you're quick and accurate enough... at least until the game starts throwing too much at you to handle without the need for powerups. I did beat the game, but I didn't four-star every mission, and the ending isn't worth it. I have my doubts that 100%ing the game is worth the time it'll take to beat it, and I personally can't recommend this game unless you wait for a rather hardcore discount; it's not worth eight dollars.