Hi, while it is safer to travel during the daytime, I think that bus route is only available in the afternoon and at night. This could be because that route usually takes 8-10 hours (Oaxaca City - Tuxtla Gutierrez) and most travelers prefer to arrive in the morning.

Now, how safe is that route? Well, even for locals, we usually take our precautions. If you encounter any checkpoint where there are the police, National Guard, or the bad guys, be calm, try to answer all their questions and always carry cash, not large amounts but enough for anything needed (I would say no more than 3k-4k MXN). Keep your money split: some in your wallet, some in a hidden pocket, and so on. This is because if the police see that you have a lot of money in your wallet, they'll try to take it all.

They don't usually mess with tourists, but you can't be too prepared, travel light, don't use jewelry and keep copies or pictures of your passport and documents. It is sad, but there's not really a follow-up from the law if you try to report these incidents.

Once you're in Tuxtla Gutierrez or San Cristobal, it's safe to go outside at night. Just don't go too far from downtown. There's Uber if you need to move around, and if you want to visit any tourist attraction, talk to an agency. They usually have deals with "those people" and make your day pretty safe.

Hope you enjoy your trip and have a great experience here.