The slag is less problematic, because chemicals are destroyed by the burning process. You're basically left with ceramics, melted glass, bits of metal and a bunch of oxides. After the useful elements (mainly metals) have been removed, the remaining slag is buried.

By ecological balance I was mostly referring to the emission of CO2. While that is an inevitable part of incineration, it is largely compensated by reclaiming the metals, plus using the incinerators to generate electricity and district heating.

As I recall, incinerators have a poor reputation in the US because unfiltered incinerators were used back in the 1940s and 1950s, causing serious pollution problems. Modern incinerators filter the flue gases and the results are clean enough that the incinerators are often inside cities.

The OP did say that the slag was processed to remove everything that can be reused. I would imagine whatever is left over heads to a landfill, but at that point it is at least mostly bath water.