I have bikes with both. The grip shifters are fine, but more difficult to maintain. The thumb shifters are MicroShift, and are excellent.
I get stiffness and cramping in my hands from time to time that means I can’t close my hand properly to grip the grip shifters. With the thumb shifters I can nudge them hawing my entire hand so I can still easily change gear.
If I wanted to build something bombproof, I would use thumb shifters. I can change the gear cables easily. I bent a derailleur a year or so ago, and messed up the shifting on my gripshift bike. I couldn’t shift as indexing was way out of whack and I didn’t have my tools. With thumb shifters I could have just switched between indexed and friction mode and kept on riding.
Easy to shift, easy to set up, easy to maintain, cheap to buy, and the MicroShift ones I have are very well built, high quality, and look the part on my ECR.