I recently went to the Riviera Maya and the experience was pretty chill, honestly. I stayed in tourist areas and everything was normal. What I did notice is that there's a pretty big difference between being in the hotel zone and getting away from it. The advice about avoiding empty streets at night is valid, but I'd say it's more important to be aware of where you're moving around during the day too. It's not like everything is dangerous, but there are neighborhoods where it's better not to go without knowing where you're headed.
One practical tip I didn't see mentioned: download ride-sharing apps that work well there (Uber exists) and always use them instead of hailing taxis on the street. Seems basic but the difference is huge in terms of safety. Also don't flash your phone, watch, or cash on the streets, like in any big tourist city. And check the local consulates or embassies before you go to have emergency numbers handy.
December is high season, so there'll be a lot of tourists around. The Playa del Carmen area you see in photos (the main avenue, the bars, the resorts) is pretty safe because there's constant foot traffic. Just stick to those zones if you don't know much and you'll be fine. Most people who travel there don't have any problems.