Alright, so, I've gotten past the fourth level, and there are GLARING issues with this game to the point that I can feel them breathing down my neck as I play.
There's mouse jittering - in a game completely about mouse control and aim. By this I mean every time and on every browser I've gotten this to run on (because Browser Unity is.. eh at best, no fault of your own) moving my mouse to the <= Left can at times be nigh impossible. I lowered my DPI on my mouse for better fine control to make it easier to make broad movements rather than minute ones to counteract the recoil and aim more specifically.
The issue I run into is while every other direction has no issue, any movement left (which I can replicate in every other screen of the game) will for no reason at times simply stop going left. Seems small, but in a game where the mouse movement is EVERYTHING, this can be crucial and extremely frustrating for a player.
There's also HUGE realism issues going on here - I get it, it's a game, I play a boatload of games myself and realism isn't the primary focus. What I mean more specifically is that it breaks any kind of cool immersion that, despite the low-poly, this game could actually have. So, what about it does this? Well, the planes, primarily, as well as the gun itself. Not particularly the abilities and the like, I'm not going to say anything negative about stuff added on top of it to make it more enjoyable/playable. I can suspend my disbeliefs for science fiction type stuff that allows for fun, interesting challenges, but the base of the game needs to be really good if not great to make it there.
So what do I mean? Well, planes don't, wouldn't, and couldn't ever bomb a town by just flying in awkward circles above it going maybe 80km/h. Seeing some kind of grouping to the planes, at least until their group is almost destroyed, would make it feel more "right", as well as having the planes come in for single bombing runs, rather than flying about dropping bombs occasionally.
I'm not going to get super specific on it, but bombing runs are far more organized than.. the mess that is those planes. Iron Rain or whatever the third level was did this REALLY well, and seeing that as the main way for bombings to be done, even by smaller planes, would be great for the realistic appeal of the game. There doesn't have to be good AI, just a believable organization level, as well as having the planes come and go as they drop payloads, rather than just aimlessly wander above the target. This, I believe, wouldn't be too hard to make possible, and once their formation is say halfway or more depleted, they could break off and do what they do at present (albeit without planes bugging out and flying into the ground for a solid 5 seconds, but that's a bug).
Next is the gun. Oh man that gosh darn flak cannon. Let's get started with the recoil, there's a very common and understandable misconception here. When firing a Flak Cannon or anything similar, the actual felt recoil by the person firing the gun or enacted upon the sights is actually incredibly small. I mean, they're made for this right? Assuming this gun is anything similar to the ones used in WWII, it's actually not going to be that hard to keep the sights on the intended location. With extreme distances, sure, but during a bombing raid, it wasn't hard to find an incoming target and follow it with the sights, weather permitting. So, why was the recoil so little? Well, the entire gun is made to be stationary. Therefore, it can be bigger and bulkier, as well as more sophisticated. The weight of the entire gun is simply so much that everything tends to just stay in place, the recoil isn't enough to cause any major shaking or jumping of any of the parts due to it being held in place. The barrels have plenty of room behind them for shocks/springs to reduce and spread the recoil energy, which in turn is transferred to the whole weapon system, not just the upper half. In fact, if the recoil was enough to make it budge, the WHOLE weapon system would slide backwards or jump upwards. That's a lot of recoil, considering a normal man couldn't lift one of these guns themselves.
One could make a case that it's similar to a machinegun on a bipod or mount, but the physics simply don't work anything like that, it's not free-floating like a machinegun would be, and is controlled by precise movements of gears and systems rather than simply the user's arms. Well-fitting (as they ought to be) gears won't budge so easily in comparison to a person's shoulder, and these systems are built to withstand any kind of recoil force, which after the springs/shock absorbers used, isn't much.
TL;DR: Flak Cannons (and other weapons like them) don't work at all like how they're being portrayed. The recoil needs to be lowered heavily.
Cutting down the recoil would go a long way to making this feel more appropriate. Maybe I'm being picky, but it just doesn't feel right when a huge gun that's supposed to be made for this can barely do the job and keep on target. Of course the recoil makes it shake a little, of course it can throw off accuracy a bit, but nothing outside the bounds of being easily controlled and handled. The game, however, portrays it as though this is simply a weapon on a mount that you press up against your shoulders and you simply have to eat the recoil.
I get why it's there for gameplay purposes, it adds a level of difficulty and uncertainty, which can be enjoyable. I just simply think that the recoil needs to be reduced quite a bit before it'll feel right. I would play more, but I'd prefer not to sprain my wrist controlling that god-awful recoil.
Fun game, solid effort, but it really needs some of the base elements adjusted before it can be a solid, consistent game that's not so insanely hard that it hurts physically to play.