As someone who got to the elite level in classic live Mabinogi using melee exclusively, I will say windmill is far from being over powered. Smash, defend assault slash and charge are your bread and butter with pet rotations to manage multi-aggro for maxing out DPS with some windmill especially in emergencies in between to avoid getting hit.
The thing to understand is the cost of 10% when your HP reaches 400+ is a huge pain to manage and you will probably spend your entire life-cycle playing the game with pot poison or being deadly both of which are not recommended at that level of difficulty especially solo.
I will say that being a melee at higher levels in the game becomes more a hassle than its worth and your fingers are going to have to be jumping all over your keyboard and mouse like a Korean MOBA player where as other classes can relax in the corner of a room and range DPS/AOE with limited pet multi aggro control.
Windmill is a defensive skill, this is not a class oriented game at least not yet (</3 Live Servers) and so if you want to get it you get it if you don't then skip it.
It will continue to seem over powered early to mid game while you're fighting enemies with low hp/low protection but its limits will quickly become apparent later in the game especially as a melee player.
If windmill were to be nerfed to the level of stomp there would be no point in using a human melee player anymore.
You forgot splash, especially with r1 smash and high ranks in other skills/stats. I can honestly say, as a giant who used a max splash range battle hammer, that i could go in hm/elite and smash something, then go stand by a mob that's not aggroed and wait for the mob i hit to get near, then smash the mob i was by for the smash splash to send them both flying. It actually does work well in tandem with assault slash>wm if they live.
WM is still pretty OP in mid to late game, but not end game. Purely because it does ignore prot for factoring crit rate, and consistent crits can be a huge game changer. End game, you'll be high enough crit it's not a big deal for most areas, but late game it can still have quite a big impact.
I also can't agree that it's a defensive skill. Several mobs have their ai reset by wm. Depending on what type of mob you're up against, you can even wm a defense, then proceed to wm again thank to the invulnerable frames messing with the mobs' targeting. Unless you can kill in 1-3 hits, you'll definitely want it for nightmare humanoids. Wm spamming something in a corner while holding a very slow weapon, can keep it locked if it has no passive defense and doesn't have like sahagin speed. All this is just solo strategies too, wm can be combo'd with others, or used for smashmill, in plenty of other ways too. It really just depends on how the individual prefers to fight.
There are quite a few tactics i've seen in melee, and it's not restricted to pure melee. A golden oldie would be smash-ib/lb-charge-smash-delay assault slash-smash-counter then repeat from the bolt part of the combo. Extremely effective on anything that doesn't have passive defense, and even if you don't do dmg you can still use it to lock down a mob, while say a friend clears out other mobs. Another less effective tactic, but amusing to do, is for giants to have knuckles, do the 7 hit combo to knock an enemy back, load lightning bolt and run near enemy, hit with bolt, follow with stomp as soon as enemy moves, and then you can actually assault slash because if you did it right, the enemy is knocked over by stomp. It's not a very efficient combo, but it is funny to pull off. To this day, think i've only seen one other person besides me do it though.
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