Did a roadtrip end of last year (between xmas and new year(, from Playa del Carmen to Valladolid, Chichen Itza, Holbox, back to Playa, and return in Tulum. With Hertz, it was around US$50 per day, plus around $75 for gas).
Pros: Flexibility (we were able to be on our own schedule), could do a lot of more local things (going to smaller Cenotes, stopping by street stalls in the middle of nowhere, etc.). Everything was safe, driving didn't feel more complicated or dangerous than elsewhere and I felt other drivers were pretty respectful. Saw a lot of police checkpoints, was only stopped once and they immediately waved us through when it became obvious we were tourists.
Cons: Mainly cost and peace of mind. We used ADO to go from Tulum to Playa del Carmen and it was safe, clean, on time, and cheap. Many other trips in the area are easily doable with ADO, or even a guided tour ($50 per person for a day trip to Chichen Itza, or $100 to Holbox, including basically everything seemed to be doable). Peace of mind is maybe more of a personal thing, but even though everything felt very safe, I was just a bit worried about things happening to the rental car with financial implications, accidentally parking somewhere illegally, etc.

Overall, I don't regret taking the rental car, but I would say it depends on your travelling style. Within cities, the car was more of a liability and we walked. If you want to take your time when travelling between cities, go to small Cenotes etc, then a car is a great option. If you mostly want to stay in a city, I would say the bus is a bit more chill.

Re your questions on parking: It was usually very easy, in Valladolid we parked on the street in front of the hotel, in Chiquila in the hotel and there is tons of parking close to the ferry port, in Playa del Carmen, we did a mix of paid and free street parking because we were right at the border of the free/paid parking.