SaraEA, ATX should be able to handle everything you listed. Any program will have its downsides, and for ATX and for what you listed, you may have to input manually for the out-of-state credits to calculate unless you pay extra for their service to calculate the credits automatically. You've probably read about the others on this forum: that the automatic backup auto-save doesn't work; the program, its files, and its backups take up a lot of hard drive space; and the errors within the program that cause it to not start or to crash such as the inability for the program to connect to the program's internal server at times (not the internet server), and other unexpected errors that cause it to lock up and for the user to lose data. Of course, that's only what I've gathered from others' postings on here. I left ATX with the 2012 debacle.
I agree with Abby and John. Demo the program, rework a few of your more complex returns from 2015 with it, and consider running concurrent programs.