These things are trash. If all you want to use them with is Arduino, or if all you want to do is display data (and not drive anything) with them, they may be fine. It's very easy to update elements on the display, but as soon as you start trying to get anything back from them it's so much trial and error it gets old quick.
The Arduino libraries, at a glance, look like they are handling all the common stuff pretty well. Anyone using those libraries should be very thankful to the people who developed them.
If you are trying to use them with any other system directly via serial, and hoping to use them to drive something, it's going to be a lot of extra work to figure out how to make sense of what they're sending. There is no standardization for the data that they send, and it's hit an miss if they do you the courtesy of using a byte or two to describe what they're sending.
I fought with one for a few hours, basically hitting buttons, seeing what it sent, and writing code around that, until I had what should have been a basic working system only to find out that hitting a button 1 time would send the string that I told it to, and hitting a second and any number of times after that would send that string and some other data. I don't know what that data was because that was my breaking point. I chucked it in a drawer to be used with some AVR/Arduino project maybe some day.
If you have component limitation, and you really need to offload the graphics and ui, they may be worth it. Otherwise, I recommend budgeting the resources to use standard displays.
If you're stuck using one, Instruction Set - Nextion , will have about 10% of what you need. It has SOME of the error codes, but even those tend to be vague.
Edit: To be fair, that page will have probably 90% of what you need, but the other 10% is just going to have to be greped out at a serial console.