Is Appian Developer a Good Job?

For me, yes!

I think only you can answer that by asking yourself some questions:

• what do I like doing? do I enjoy solving puzzles? do I get satisfaction from seeing something I've built working? does is give me pleasure make other people happy by giving them something that makes their lives easier? do I enjoy continuously learning?
• do I have the necessary skills - both technical and other e.g. communication, management, eye for detail, collaboration/teamwork?
• does it economically work for me? is there another career that'll provide me the the income that I need to do the things I want to do?

(I'm hoping you can see that it's not just about the money!)

According to my opinion, Yes! Appian developers are in demand. 
If you want to participate in Appian certification exams, I recommend taking Appian practice test material for successful exam preparation. These practice tests will help you prepare and pass the test with ease.

Yes, it is. Appian is quite popular and it is in demand.

I've made no secret of the fact that joining an Appian dev firm (just over 12 years ago now) literally saved me from a dead-end job as a base-level hardware tech for retail computer equipment and loosely-related items, which was interesting in its own right but tended to be very dirty, thankless, and with really unpredictable hours.

The caveat though, is I was already interested in software development, and though I had no professional experience to speak of, I'd trained myself in some basic programming while in high school, and had gotten a degree in computer science (which was, since, collecting dust).  So I'd warn that it's not just for *anybody* - you need to be interested enugh to learn, you need to be a certain degree of fearless to try out things and troubleshoot solutions when you can't figure stuff out at first, and in most cases i'd wager that you need at least a small predisposition towards software dev or at least algorithmic thinking, in order to not have a bad time the first time you end up needing to implement a complex expression rule to do some sort of data transformation.

But for anyone who fits most of that bill?  I'd say yes, it's a field that's well in demand at this point, and will ultimately pay off.

It perhaps depends where you coming from:

non IT: the first question should be, should I

a) becoming a developer in general
This you need to answer yourself independently, in my mind. 

with the decision made or dev background with e.g. high or language like C#,python, java etc or another platform
b) if you want to become an appian developer

I really really just like appian. This platform has a high quality and you can do a lot of things with it. I cannot imagine having the same fun as ....java dev?
The barrier to entry, to become a developer is lower in my mind as the learning curve is pretty nice.  But you need to be curious as a lot of things are changing. People who did the last time something with appian in 2017, need to invest quite some time to come back in track as a lot have changed.

The market is currently pretty clear. More demand then available resources

Recommendation: get a community edition environment, do some courses and see if you enjoy learning it. That fun is necessary to pull through hard times like a stressful project or being stuck in a learning lesson.

I advise using the most recent Appian practice test materials if you wish to take the Appian certification exam. You can prepare and ace the test with the aid of these practice exams.