I can't help you with info about AZ, but my nephew did local and regional NTIS last 2 years. Since there wasn't much recent info on here when I searched, I'll chime in anyway since I can speak to the national event in Cary. OH Valley switched leadership fairly recently (I think after 2015), its run by a guy out of Pittsburgh who I thought was fair through the local and regional tryouts. The best kids from our local tryout received invites to regionals, and the ones that weren't good didn't. Nephew made it to the regional tryout both years, but he got sick at regional tryout the first year and had to leave early. So we did local tryouts again last year and made it to Cary, NC.
As others have said, players selected for the extra workout session in Cary seemed to be pre-selected. Some because they were already involved in the USA program before (which was the case on our regional team), or some for other unknown reasons but most likely reputation from regional NTIS coaches who know the kid from their travel program. I think it is possible that if your kid is a true stud that hasn't been on their radar, he could have a real shot at invite to national team tryouts. I do think they try to put together the best competitive team possible and wouldn't pass over a truly star player. But some of these kids have been participating in the USA system for several years and they always seem to get re-selected no matter whether they do good or not in Cary.
NC is not going to be a cheap trip from the southwest. If you have the disposable income, go for it. The kids all seem to enjoy the experience even if they don't go any further. As others have said, it's pretty cool memories if you can afford it. I agree with others who have said the NTIS series is a money maker for the program. They've gotta fund it some way so that doesn't really bother me.
Also keep in mind, there is another "path" to national team tryout invite. Son could do the USA Team Championship tourney which is the other "path" to national team tryouts. We didn't know this until recently, but if your kids travel team isn't interested in doing that tourney, your kid can sign up for that as a free agent and be put on a free agent team. That tourney is the equivalent of the entire NTIS tryout path. If you have specific questions, PM me and I'll be glad to answer.
Just know going in, it is a very selective process (some parts fair and others maybe not so much). One of the top 10 PG ranked 2021 players (who we play travel ball with) tried out via the tourney path for 4 years before he was selected for the national team. So they are definitely looking for the best of the best.