Last time I used traveler's checks was 1971 and it was such a pain I would never use them again. Finding a place to cash them? How much to bring? Where to get more? Slight variation in signature and they would not cash them. Of course this was a very long time ago but to get cash in San Miguel de Allende I had to go to my aunt's bank with her and they put the funds against her account until the checks cleared. And yes I know there is an American Express office in SMA.
Yes, we have had problems with ATMs in Mexico. We now have personal and joint accounts at two credit unions with debit cards for both of us with different numbers on all accounts. We use our computer to watch our accounts like hawks at all times no matter where we go.
Yes, we have credit cards too but we go to places that do not take them so we find we use a lot of cash. We would rather give a gas station attendant cash than one of our credit cards.
The problems we have had and have seen others have has had to do with machines that will not give money to certain cards. We use machines at banks whenever we can. We have different cards to use and can change our PIN or cancel a card at any time. You can be robbed anywhere you go in any number of ways. Being careful at all times usually works.