It won't give an accurate representation of overall frequency response, but it will allow you to pinpoint frequencies.
I’m under the belief that iPhone is a decent platform for an rta app because the phone models are very cut and dry and completely standardized. The android os is the complete opposite with numerous devices of greatly varying quality. What’s it like 13 different iPhones with only about the 6 most modern still in use, so the app developers can predict the microphone characteristics and program accordingly. I remember looking into this and walking away with the belief that an iPhone rta is actually “pretty” good for casual use.
There’s seemingly a fairly regular stream of posts asking “where to crossover (insert whatever)” and I almost always suggest a smart phone rta because at the general level of understanding that yields such questions, a simple look at obvious dips and peaks is great.
I only mention this because it’s easy for people to see the gurus on here measuring with calibrated mics and rew on very refined systems and think “all that measurement stuff is crazy and out of reach so I’ll just cross my fingers and turn knobs”.
I know you already kinda said this but I honestly think more people getting into sq should be using some sort of phone measurement, preferably iPhone but anything is better than guessing, imo.