Why do some recommend uncoated asprin instead of coated if using for cancer prevention?

No benefit: There is no persuasive evidence that aspirin, whether coated or uncoated, reduces the risk of cancer. There is a small, but not zero, risk (to stomach and kidneys) from daily aspirin. For people who are at high risk of stroke and/or heart attack (because, for example, they've already had one and are at risk of another), the small benefit of daily aspirin exceeds the small risk.