Can follicular adenoma thyroid nodules be serious?

Yes. A follicular adenoma may be a wolf in sheep's clothing, because cells from follicular adenomas and follicular carcinomas can look the same. Other follicular adenomas may have almost no chance of being cancers, but overproduce thyroid hormone, and need treatment to prevent harmful effects of hyperthyroidism on the bones, heart and other tissues.
Usually not. Most follicular adenomas never require additional treatment. But some continue to grow and require surgical removal. Follicular thyroid carcinoma can also be "hiding" in an otherwise benign adenomatous nodule.