I agree playing with no nail on the thumb has a nice effect and it is very useful. On the other hand, I think it is better to learn playing with thumb nail, so you have the option to play with nail or without it, according to specific musical necessity. I think it is a pitty modern players don't use as much a good variety of tonal colors, so you hardly see them playing thumb with no nail. But some great players still use varied thumb attacks, including no nail.
If you are used to playing with thumbflesh only, playing with nail recquire a different hand angle which interferes with the other fingers and it is certainly not an easy task to find a good position for all the fingers together. A good teacher would be the best shortcut.
With all that said, the great Abreu duo didn't have thumbnail in their first recording, and it is great, I didn't notice until someone told me that:
But Sergio said it was a mistake not to have it, he admited it was a waste of time.