Whoever has it worse, each side will now be able to produce countless arguments, but if you add it all up, you get this result:
The night elves lost three territories first to the Horde, one permament, one floating, and one reclaimed.
The Horde lost exactly one area, Tirisfall, while the other areas of the Forsaks remained intact, as we know. However, their main base went down with it, Undercity.
So I would say here: Night Elves lost more territory in BFA.
Next point: population.
The Forsaks were able to evacuate their population, but later on - or so I think - Sylvanas heated them up. I can’t imagine that she heated up everything except her forsaken people, no, that would be inappropriate.
The night elves could only save a part of their people, a relatively large part died, cruelly.
That’s the way some of the population goes to night elves, but only in the first moment, in the second moment it will probably have hit the forsaks as badly, because, I’ll explain now, let’s get to the next point: Army
The army of the night elves was barely affected, largely intact, and had been a challenge to the Horde even before this , it was now a monster with Tyrande and Malfurion, which beat the Horde back bit by bit, no matter how many hordetroops were here…Tyrande alone destroyed entire Horde camps in an blink of an eye, Malfurion became the terror of Darkshore, and the Darkmoon army was a powerful and strong army, a veritable monster.
The main forsaken army had suffered serious losses in Tirisfal, many died there, and finally the entire Horde army was crushed as it rushed to defend undercity. Later, they were probably able to fill their ranks with civilians, but this against trained soldiers of the night elves, whose civilian population was already a challenge to the Horde, was more likely to lead a lamb to the slaughter than a balanced fight.
They then, of course, raised night elves, but even they eventually lost to the black moon army.
So: Clearly, the Forsaken lost more here.
Hero characters:
The Alliance lost a lot of this addon, but no Hero characters.
The Forsaken lost Sylvanas and Nathanos:
So this goes to the Forsaken as well.
So both lost a lot, but in completely different things.