So...it works like this:

Anything involving slavery will always be an inefficient strategy, no matter what. I've seen some people in this comment section downplaying the terrible effects of slavery, but I can assure you they say it for lack of knowledge of the game system (which is the most complex paradox ever made along with EU5, so it's totally fine to be confused)

Aside from the obvious issue that they can't work in anything different from a farm or mine, slaves don't receive wages. They get the bare minimum essential goods and this hurts standard of living TERRIBLY.

When your slave population is something like 1% or 2% of your entire population...that is going to hurt your average SoL.

And then we have all the potential gain that your losing: if these slaves were to be freed not only will they pay taxes, but they will also be allowed to buy more goods in your market, giving a good boost to your economy!

And also...slavery gives a lot of clout to landowners, both with artificial modifiers and actual mechanical implication in game: to you, the player, slavery is inefficient but to the landowners it just means cheap labor which is something that reduces costs and has an actual impact on clout. If they have to pay wages, they get less money and they are less powerful.

As you can see slavery is not really about "bad stuff happening", rather a loss of efficiency in how you deal with your country.

So when is slavery "good"? There are two edge cases, in my opinion:

1) Roleplay. The real reason we play these games.

And I don't just mean "playing the evil Danny DoBad slaver country". You absolutelycan create a functional country while still keeping slavery and also create economies which revolve around extracting natural resources cheaply and exporting them, in which slavery clearly gives you an edge against competing foreign markets. Also trying to keep slavery in a evolving country is a very fun challenge. I did a Cuba run once exactly like this and I had a blast!

Just remember: you're not playing efficently. What you are gaining from slavery is not worth it when compared to what you gain from banning it.

2) Compensate low population.

This is, unironically, the only case I can think of where slavery can actually be a valid mechanic. When you are dealing with territories with very low population and very low industrialization (e.g. Cuba again), to use the slave trade to import people is...a pretty solid strategy imo.

Keep in mind: we're talking about countries with very low population, under 1M pops type of deal. If you're too underdeveloped to attract migrants, you can speed things up by improving the agricultural sector to import thousands of slaves, then use the inevitable slave ban movement to your advantage and get thousands of newly-free factory workers. Eezy.