Is mobile data (eSIM) more reliable than the Wi-Fi at my Riu / TUI resort in Sal?

Yes, 100%. Wi-Fi in most all-inclusive resorts on Sal and Boa Vista is notoriously slow, congested, and 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, most importantly, on the beach (like Santa Maria), where hotel Wi-Fi never reaches.