Will it crack if my slab cracks underneath?

Epoxy is harder than the concrete it sits on, but it follows the substrate. If the slab moves, the resin can crack along the line, so we repair the slab first: we open hairline cracks, fill them with epoxy paste and let it cure before the main coat. For floors with live structural movement we flag it when we quote and talk through a flexible system instead. If we suspect the slab is still moving, that is a question for a structural engineer to assess before we quote or lay anything, rather than something a floor coating can fix.