Read the fare class bullet points
Basic economy varies by airline. Some allow a full-size carry-on; some do not. Some block seat selection until check-in.
Changes and refunds
Many basic fares are use-it-or-lose-it for voluntary changes. If life is unpredictable, the upgrade to main cabin can be cheap insurance.
Boarding and overhead bins
Late boarding groups mean bin risk. If you must have overhead space, factor that into cabin choice.
When basic economy is fine
Short hops, solo traveler, small bag under the seat, fixed dates—then the savings can be real.