Does vitamin b12 deficiency cause more serious complications than iron deficiency?

Both deficiencies will cause a reduction in blood cells (anemia) which can manifest with fatigue. B12 deficiency is potentially more serious as it can trigger damage to nerves in the body resulting in numbness, difficulty walking, and weakness if left untreated.

Yes. Both deficiencies cause anemia, but B12 deficiency also affects the nervous system, starting with peripheral neuropathy.