Is mobile data more reliable than my all-inclusive resort's Wi-Fi (e.g., in Negril)?

Yes, 100%. Wi-Fi in most all-inclusive resorts (like Sandals, Riu, etc.) is famously slow, congested, and often only works in the lobby. A personal eSIM gives you a private, fast 4G/LTE connection that works everywhere, in your room, by the pool, and on the beach, where hotel Wi-Fi never reaches.