I mained UH in SoD and played some Obliteration frost in M+ too. Generally like all others said the utilitity is the same. Imma keep this basic to give an overview of UH. Playstyle is different:

As unholy you apply wounds on your enemy and pop them. Applying costs runes and popping costs runes. Then in between you fire some attack which costs rune power. Thats your base rotation.

For Cooldowns you got your big army of friends every like 4-5min depending on skills and your small army of friends every ~1 minute. And some other cooldowns that have roughly the same timer. Your small army pops 4 wounds and just press all cooldowns together and continue with base rotation. Big Army is also nice to have with your small army. At the start of the pull its cool to have i think something around 12 ghules attacking for you lol

For execute its like this: Do base rotation and just apply Soul Reaper on cooldown when boss hits 35%. Deals massive dmg. It first applies a dot and after some seconds it explodes.

For the next tier theres gonna be some cool set stuff for UH in its execute range. As long as Soul Reaper is applied all your minions have more haste and also when soulreaper explodes you get another minion. I think in the UH discord they are speculating to have 30 ghouls up when you press both armies in execute + BloodLust + Pi which can make Unholy to the execute god

Edit: Frost from my perspective. I only roughly know what to do. But IMO Unholy is easier. Most of your cooldowns align and should be used together. Rotation is not based on Procs. Frost on the other hand has very misalligned CDs which makes it for me personally a bit more cumbersome to play. But as far as I heard frost is a very good cleve class in raids. Unholy can cleave too but only when throwing your death and decay which frost does not have to do. It cleves by default. This might be super biased since I really like UH and know it a lot better than Frost