The way I think of Winmillion, is as a defensive Cutter. Everything Pery and Fehzor said except for the projectile are words more suited for the Cutter. The Cutter outperforms the Winmillion in "burst damage" and due to having a 5-attack pattern, it doesn't have that long 3rd attack vulnerability that the Winmillion has. Also, Cutter has 5*, so it has better damage and has more interesting looks.

Where I believe Winmillion kind of shines, is a defensive/retreating playtyle utilizing mainly the 1st attack. Against anything more than one enemy, the Winmillion full attack combo becomes much more dangerous than it already is(if you aren't using auto-target). I believe the full attack combo is only useful when you're already using the Winmillion and you see a safe target to pick.
I space my 1st hits and try to hit the tip of the sword and the bullet on enemies as I walk backwards or strafe around. Though the Cutter can also do this, it's not as effective imo because Cutter forces you to be a closer to reliably pull this off, while Winmillion has that extra range which is less stressful to work with.
Let me clarify something. You could use a DA/Troika for a defensive playstyle, but they're not really good for retreating. With DA and Troika you need to have that space to either chargespam or safely swing your sword. And if you're chargespamming, you don't need to retreat anymore because you pushed the enemies forwards/away. With DA/Troika you can move side to side and rely on the large swing-arc to hit enemies, but with Winmillion if you aren't using Auto-Target, it's easier to hit enemies with the bullet if you keep retreating. ugh... You could also move side to side, but it's pretty dependent on the situation about what kind of enemies you're kiting with the Winmillion. Though in general it's better to retreat in one direction.
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Winmillion charge attack is also more suited to a defensive playstyle. You could use it semi-effectively against Kats and Greavers as they rush you (Calibur would do more damage and knockback, if you're into that sort of thing). You could also use the charge attack against things like Giant Lichens and Lumbers and stuff, but you could use a DA and wreck them harder. I rarely use the charge attack because usually my other weapon(I don't use slots) usually has a better charge attack for the situation. Winmillion charge can handle both of these situations, but I'd normally use a Calibur and a DA instead of considering Winmillion charge as an option (because it's really subpar in comparison to these two).

Winmillion(for its regular attacks) is kind of that weapon for the player who doesn't use slots and would like to have options against all enemy types(like other normal-type swords) and wants a weapon in between close and mid-range(aka around edge of sword-swipe-range).
Beyond that range you might as well use a Valiance or Iron Slug because you're doing shit damage with the bullets, and before that range you might as well use a WHB/DVS if you're into the multi-hit and speedy attack. If you have weapon slots, there's not much point in having a Winmillion because two weapons can vastly outperform the Winmillion for the things it does. If you're bombing you might consider a Winmillion, but I'd go with a Cutter to deal with Greavers and blocks. If not Cutter, I would consider a gun to deal with gun puppies because bombs are usually a better retreating option than Winmillion.
Winmillion is the most in-between weapon in the game...

tl;dr: With Winmillion, you could do a variety of different things that Calibur, DA, guns, and Cutter can do, but it isn't better than them individually.
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The other way I used to use Winmillion is as a combo starter for heavy swords. I would do 1-2 attacks with the Winmillion, and switch into 1-2 attacks with some heavy sword. I don't do this anymore because I might as well use a Cutter in close quarters, and just use pots or Drakon to deal with switches.