In terms of losses within the lore, the Horde wins out over the Alliance, but the damage done to the Horde itself in terms of characterization was just as marked and arguably worse.
The Horde’s losses were all temporary or surgical, there was very little actual destruction, and the loss of Undercity was at the hands of Sylvanas herself. I’ve said this before, but if you think there isn’t some Blight cure Macguffin waiting on the wings, you haven’t been paying attention. Undercity in terms of infrastructure was nearly untouched, once the Blight is cleared. The only real loss there was Brill, and the Forsaken still hold/held all their other zones.
Dazar’alor wasn’t a sack, there was no massive large scale destruction of the city, no looting. The king died, and some important figures, yes, but in terms of losses logistically it’s not exactly repayment. Zandalar immediately gained a more powerful queen, and one who was more willing to work with the Horde. The loss of the Golden Fleet was a loss, that much I’ll grant you.
But one should keep in mind that aaaaall the forces Sylvanas had, the ones that Anduin said made up ‘the only force possibly capable of defeated N’Zoth’, implying that her army was still more numerically superior to the combined Alliance and rebel forces… didn’t go anywhere. Some loyalists were carted off, but the implication is that the Horde united now has the more powerful force. So logistically speaking, and in terms of territory and infrastructure, the Horde ‘won’. Especially since they’ve now gotten rid of the leader who was apparently trying to get them all killed.
One point in the favour of Alliance wins, however: The Kul Tiran fleet wasn’t all destroyed. The number of ships in the cutscene wasn’t nearly enough to be the entire fleet we saw at Boralus. And even if it was, the Alliance still has Anduin’s ship, which is more powerful and advanced than any other naval vessel. So Alliance did win out there.
The Alliance ‘won’ in Arathi but I highly doubt the zone will go full Alliance even in lore, Hammerfall will remain. Darkshore was retaken, but the entire zone of Teldrassil is flat out gone, and we have no idea as to the status of Ashenvale, because the last time the Horde ‘lost’ to the Alliance, they were gifted the night elf lands they’d taken for the sake of peace.
So, looking at it objectively, the Horde ‘won’ the war and are currently in the better spot. They are, however, in a far worse position when it comes to their characterization and story, if only somewhat because boy did things go off the rails after the first four months or so of BfA…