Kingdoms in Empire

Depends. There is less micromanagement, which is either good or bad depending on what you want to do. Pretty much the same as dukes vs only counts, except one tier up. Like you said, small ones such as Serbia don't make so much difference, but creating Greece or Anatolia as Byzantium and giving it away probably wouldn't be smart.

It's rarely a good idea to have king vassals in your empire (or create unnecessary kingdom titles), unless you want more drama. Small ones like Serbia don't matter much, but it's not worth having a powerful vassal who's stuck with an extra -25 opinion toward you. Whereas upgrading to dukes vs all counts will enable tech point generation, going from duke to king offers very little reward; the extra layer of bureaucracy will cut down your feudal tax income even further. If you simply want powerful vassals and less management, then just create archdukes whose duchies aren't within the same de jure kingdom and save yourself from that unnecessary -25 opinion malus.

I personally like to create the king titles and hold them myself I am still a little bit unsure if this is a good idea or not but would never give them to vassals really though it does seem to speed up the game to have a small number of vassals rather than a large amount.

Benefits

Extra prestige
less chance of independence war as most vassals become dejure

Costs
most people will dislike you as they want the king title.

Kings though always join factions and so are a pain even if its something like the king of Galicia or Castille.

The only reason why one should give away kingdom titles under an empire is as previous members said to create drama. Because when you are an empire its pretty much game over for the rest of the world nobody can stop you

You'll get a better control over your empire by using kings and larger retinues and more money.

Also if things go spiraling south, with lots of rebellious kings, just jail the lot of them, strip their titles and banish them from your lands. You'll get a bunch of money and you can reinstate new kings that love you. Granted every duke and count will hate you with a passion, but that doesn't matter one iota, because you don't deal with dukes or counts. The kings do. And they love you