China has to be careful. The US is well aware that the PLA is optimised for striking within the First Island Chain.
As for the SCS, given the utter failure of Chinese air defence equipment in the recent Venezuela operation, the PLA's fortified islands in the SCS are not so much bastions of strength as pockets of extreme vulnerability. China, as a land empire, is fixated on controlling territory, and will squander significant resources defending these isolated bits of artificial land in the SCS.
As for the atolls themselves, they are susceptible to attack given that their defenses, however impressive on paper, probably don't work nearly as well as advertised. Once their air defenses are eliminated, then it's a small matter to knock out their power generation and water supply. In a conflict, the USMC is all but certain to seize them and use them to deter Chinese naval forces in the SCS.