In my opinion apps are not the way to go for either. Here is my reasoning.
Based of your description and my experience with some apps, these can never provide meaningful feed back on you technique or issues you may have with your body, posture, pressure, etc. Not to say that AI and sensor integration can't evolve that far, but it's not there yet. A real human who has been through the training can interact with you and correct minor issues that could lead to major issues years later. Like thumb over the neck for guitarists. Proper bowing for a violinist, etc. Some of these issues can be corrected by sight, some require touch.
Learning to play an instrument correctly requires more than just getting notes in tune.
However, most instruments I can think of are "external" to the body. Because of this you can, to some degree, get the correct posture by viewing videos and other inputs.
The voice is "internal" to the body.
My experience is as a sting player since age 4; violin, classical bass, guitar, and may other similar instruments, mandolin, dulcimer, etc. I recently began classical voice lessons. The process is completely different (at least in my opinion) from guitar or violin. Your teacher can't stick a finger in your throat and check the tension of the vocal folds, or tell if you're holding your breath or releasing too soon. There are important factors in vocal training. There is so much to learn to sing correctly, and not cause damage to your vocal chords. I would be surprised if an app can help with that. Rather that just being in tune, if the app could determine if you are stressing your voice by analyzing the quality of the note that would be meaningful. But just giving you an in/out of tune mark is meaningless when it comes to true vocal training. There are times when the quality of the voice is more important than exact intonation. Of course over time you want both to be correct but singing in tune with bad technique could lead to cysts on your vocal chords and other serious medical issues.