Any 0% tax deals won't qualify for EU participation exemption regimes so that offer can't attract a registration of a legitimate parent holding company - if substantial revenues originate from Europe.
For smaller islands like St.Kitts and Nevis, 2.5% CIT without conditions for all IBCs, and with some substance requirements beyond patent registration is starting to look like the new floor.
Competition-wise, territorial tax deals are too common in the region and available in Europe (even in France for businesses) hence that offer makes no sense for late adopters. For St.Kitts and Nevis, I see an edge in the 2.5% CIT offer, but also in a complete exit from this game to free up some reputational capacity in Europe and elsewhere to focus on passport sales which is another contentious topic: at least it won't be known as a tax haven any longer.