Slabs are actually just an emergency solution. They simulate clinker where there is none.
I wouldn’t phrase it so harshly,
They do not save any labor costs compared to real clinker.
especially since they actually are real clinker, just cut into thin slices. Slabs are a product designed for the application case where clinker is desired but for various reasons was not planned or should contribute less to the total thickness. So for retrofitting or also in new construction as an aesthetic layer. Slabs on plaster are nonsense, as they are supposed to be an alternative surface instead of plaster and are used exactly where the issue of wall thickness has already been fully satisfied monolithically. Slabs as a facing shell do not work because you can’t masonry them, but only more or less "tile" them. Theoretically, you could mount them on ETICS as a curtain wall, yes, but: for that you would at least have to find the fool who guarantees the durability of this construction – and of course the suitable adhesive. However, ETICS is not intended as a curtain, but as the “E” already says, for mounting directly on the structural masonry shell. For that matter, in a new construction project I would choose the material of the structural masonry shell so that you can dimension it slimmer thanks to ETICS. Monolithic
plus ETICS is, in my opinion, subject to a tax on stupidity!
Therefore: if ETICS in new construction, then in my opinion more sensible with clinker as a facing shell, and the structural masonry shell dimensioned accordingly. The S in ETICS stands for “system” in the sense of considering the wall assembly as a whole. But since the combination possibilities are varied here, and those with which the installer has experience are clearly much less varied, one ends up again with the question of what the installer’s "answer" to the stone mantra is.