The ADO buses are clean, with air conditioning and bathrooms. Just be careful and keep a good eye on your bags and any valuables, because theft is common in the buses and at the bus terminal. You’ll also want to show up to the bus station early, because the check-in process on a busy day can be tedious and messy.
The main drawback of going to Chichen Itza by bus is that you’ll have 5 hours to walk around at the archaeological site, which may be more time than most people need. One workaround for this is to instead take the bus from Cancun to Valladolid, where more departures are available, and then take a colectivo (shared van) to Chichen Itza.
If you still have extra time left over, you could also hire a local taxi to take you to one of the cenotes near Valladolid while you’re at it! That way you get to see more places in the same amount of time.
For Chichen Itza, keep in mind you will still need entrance tickets to visit, even if you go without a tour. It’s a good idea to book your Chichen Itza entrance tickets online in advance, that way you can skip the line when you get there and spend more time touring the ruins.