A single Li-ion cell can Swing from about 4.2 V when full to 3.0 V when empty, which is fine for parts designed for that range but dangerous for strict 3.3 V ICs — 4.2 V can exceed their absolute max rating and kill them. If you’ve got mixed 3.3 V and 5 V logic, use the right regulator (buck or LDO for 3.3 V, boost for 5 V) and add proper decoupling and level shifting. It keeps your chips safe, your logic stable, and avoids turning your portable project into a post-mortem project.