You know, that's what we thought of more than a few islands in the Pacific.
The fortifications were most likely there, you just didn't (literally) trip over them (which is how you'd find them).
We underestimated the method the Japanese intended to fight in a number of cases ... but we bombarded the hell out of what we thought was there.
My father was personally, directly involved as an officer with a flotilla of LCI's.
Ultimately the Japanese spent less time on immediate shore forticifcations and more time on inland fortifications where they could maximize their attrition of our forces once they landed. That happened over and over in the Pacific. Tarawa was one significant departure from this ... but in most others their after-the-beach plans were carried out to significant effect ... albeit in almost all cases they suffered much higher casuality rates than we did and neither method prevented the inevitable.
Taiwan, in a simialr fashion, has spent much more on inland fortifications in the mountains (2/3rds of the island is very mountainous). No doubt, Taiwan will be a tough nut to crack and hold. The very attempt by the Chinese, requires that we not be there to interfere as a given and that is the principle point IMHO.