This editorial on Delaware corporations? https://a16z.com/were-leaving-delaware-and-we-think-you-shou...

Another view, https://handbooks.clerky.com/startup-incorporation/where

  Delaware is widely regarded as having strong protections against personal liability for corporations. Some advisors say Nevada has better protection against personal liability. This is arguably true, but the differences are very unlikely to be relevant to founders trying to build a legitimate business. Some people have also observed that Nevada has an adverse selection problem in that their unusually strong protections attract bad actors. As a result, it's possible that if you incorporate in Nevada, you'll be inviting closer scrutiny.

It is quite possible that it was tax neutral for them personally.

Cayman Islands has other costs and risks - depending upon your context.

AFAIK getting accounts completed and filed is expensive in Caymans.

Professional costs in the Cayman Islands are hideously expensive (from my extremely limited experience).

Additionally, FYI, a Cayman Islands company needs a local Cayman Island director which costs thousands per annum.