Hard? It's not even remotely hard. I don't know why people automatically assume annoying == Hard. Annoying means it's annoying, not difficult.
The player has minimal ability to attack vertically. The game abuses this by making bosses jump spam like a rabid bunny and spam projectiles from all directions, often putting you in positions where dodging is nigh impossible. There is also no iframe dodge whatsoever so good luck getting past literal walls of projectiles.
Once you get all the range upgrades and ability to reflect all projectiles, it becomes somewhat more tolerable because you can at least roll over most trash mobs, but the bosses are still absolute trash to fight. The game goes overboard exploiting the player's inability to attack vertically to the maximum which doesn't make for a particularly fun experience IMO.