I agree slavery is very strong, I rarely switch out of it until the industrial era. It used to be quite a dilemma when to switch into it, when happiness is generally the critical issue in your empire. However with the current system where there is no pop unhappiness until size 4 it is less of an issue, as most of my cities will be happy at size 1 with slavery and will not be happy at size 4 without slavery it does not come at much of a cost.
When it comes to switching out slavery has a production and gold advantage over the other options (the gold multiplier is unrivalled, and to get the production multiplier of guilds you need to build the guild hall), and comes with happiness and stability penalties. Once you have monarchy the main solution to happiness and stability problems is troops, and the 2 impediments to troops is production but mostly gold.
I guess what I am saying is that slavery has an inbuilt solution to its problems, and so is rarely a bad choice.
When it comes to switching out slavery has a production and gold advantage over the other options (the gold multiplier is unrivalled, and to get the production multiplier of guilds you need to build the guild hall), and comes with happiness and stability penalties. Once you have monarchy the main solution to happiness and stability problems is troops, and the 2 impediments to troops is production but mostly gold.
I guess what I am saying is that slavery has an inbuilt solution to its problems, and so is rarely a bad choice.