I think I know what you mean about Boss 3, with the close range jab that seems to required slightly quicker timing for counters.
I was found of Boss 5's reaction whenever you parried his special attack. On a side node, I'd like to mention that his ranged attack where he holds his sword up and summons projectiles has a change to glitch out where the projectiles spawn infinitely and he disappears from the level, requiring a level reset.
I'd be happy to see more bosses along with a Steam release.
I took another look at Boss 4, and the reason why I struggled so much is two-fold.
First, Boss 4 has a three-to-four hit combo where the last hit is unblockable, requiring an attack parry rather than a regular parry. But the thing is, there is very little room to figure out how many swings he is going to use, while Boss 5 will always use two axe swings before an unblockable attack.
In addition, Boss 5 has visual feedback when you successfully parry his third axe swing, while the feedback for parrying Boss 4's attack is present but far more subtle.
