Strobe tuners are brilliant. they're very different than digital tuners in mechanism. I apologize if you already understood this.
The "reference" in the strobe tuner is a the RPMs of a spinning disk. There is a specialized drive motor and associated control circuitry to hit a very precise RPM. Behind the disk, a series of small neon bulbs are activated to flash based on the waves of electric current that are the sound signal coming from the microphone. If the flashing is happening at the speed of the rotating disk (or twice that rate or 4 times, etc..) then it looks like the disk is standing still. If it's off just a tiny bit because the note is sharp or flat, then it looks like the disk is slowly turning to the right or left, depending. The unit can be calibrated by using an internal calibration circuit that sets the strobe flashing at a fixed value (probably 440Hz) and then a calibration knob can be used to fine tune a correction to the RPM, but ever since I sent this thing in for a factory refurbish, there has been zero need to adjust it.
Here's a video on how a strobe tuner works:
Basically, what it's doing is creating a real-time visual representation of beat frequencies between a played note and a reference frequency (implemented as a spinning disk RPM). I believe the apps that are "stobe" type as mentioned above also use a similar strategy, combining signals to display beat frequencies, but they're digital simulators of this otherwise analog technique.
Contrast this with the mechanism of a digital tuner which reads the frequency of voltage peaks for a short period of time and then calculates and reports the resulting averaged rate as a note name +/- a correction in cents compared to a library of reference values... then it updates. How good it detects with a microphone or vibration sensor, how it filters the signal, how it handles response speed versus accuracy, how fast the display reacts - that's all proprietary stuff and I'm certainly not qualified to critique. For the smartphone apps as opposed to dedicated tuner devices, I would think there are probably additional variables that include things like the potential for magnetic pickup of pocket dust into the microphone hole, or muffling from the different types of protective cases. These are things that make generic comparisons of apps potentially problematic compared to the individual end-user's personal device.
For my purposes, it's all good. I used the Guitar Tuna iPhone app last when I most recently restrung my guitar. It was very helpful as I brought each string up tension through the scale because it graphically showed me the rise in tone I get with each twist of the knob. My clip-on can also do this but for me the line graph display on the phone screen is more intuitive. Side by side, when the strobe tuner shows me on the mark, so does the clip-on and so does the app. However, the clip on doesn't react much to the response compared to the strobe. The Guitar Tuna app demonstrates note wavering, but the response is blunted compared to the strobe tuner. I have another app called Tuner Lite with a simulated needle gauge, and it seems over sensitive to the point of being difficult.
I did play with some online toys at one time, like a web app "tone generator" that plays a pure sine wave tone at a given frequency that you can adjust. I found a couple of versions and was mildly annoyed to find that when I played them simultaneously at the "same" frequency of 440, I heard HORRIBLE beat frequencies. Obviously, they did not agree. Similarly there's YouTube implementations of recorded tones... and I begin to question exactly what the gold standard should be.
At some point, though, this goes beyond what's relevant for me and my Sor Opus 60 studies, and onto a tangent.
My belief is that this road could easily lead me to microtonal madness! I recognize that there are inherent imperfections in the equal tempered scale to start with, and predictable intonation challenges with fretted strings. For this reason I prefer to do a quick open string tune with the clip-on, or relative tune by ear at the 5th fret, accepting, for now, some of the roughness of sonorities between notes at higher frets as part of the natural sound of my acoustic instrument.
"If I have seen further it is by standing on the shoulders of giants." -Sir Isaac Newton
Armin Hanika 56PF