I've compared my fancy Korg multi-temperament orchestra tuner to various tuner apps available for Android on my Nexus 4, and I prefer the box every time.
It's got much better sound detection, and because it's a fancy (and expensive) one there's no issue with support for reference pitches and temperaments (although all I regularly need is A440, A415, equal temperament and Vallotti's temperament).
However, I suspect if my phone had a better microphone the apps could be just as good
and they're certainly fully capable of producing what seem to be accurate reference tones to tune that way.
So really it's down to your experience, your phone hardware
and if you've already invested in a tuning box with the capabilities you need.
If you've already got a smartphone and its microphone is up to the task, an app is going to be a lot cheaper - provided you keep it charged, anyway!
music.stackexchange.com/users/1479/Matthew Walton
I personally experienced no difference.
If the app is able to pick up your violin's sound, its frequency display will have roughly the same precision as a tuner, tuner/metronome combination.
Advantages of an app: always at hand often more flexible (tuning frequency, display variations) Advantages of a device: may be more intuitive to use no hesitation to hand over to fellow musicians cheaper to replace if dropped to the floor and crashed unshared power source, so better chances to be still sufficient (kudos to Matthew for that)
My favourite Android (newly available for iOS also) app is PitchLab, but your mileage may vary.
music.stackexchange.com/users/2600/guidot
One advantage of a dedicated clip-on tuner is that it can detect vibrations coming through the body of the instrument.
A smartphone app uses a microphone that hears not just the instrument, but also lots of other ambient noise.
The clip-on tuner can use the cleaner signal coming directly through the instrument, and so may be less prone to responding to ambient noise or may be effective in a noisy environment in which a smart phone app would not be able to cope.
music.stackexchange.com/users/10638/jbyler
The microphone of the smartphone is optimized for speech, not for music.
Its supported frequency range is the vocal range of the average human talker, not a professional singer.
As narrow as 300 Hz to 3500
Hz only was declared "optimal" in the past, while now it may be wider.
Hence the microphone may not be picking very low or high frequences well enough.
From the other side, if the instrument is loud enough or its not an 8 octave piano after all, this still may work.
As long as the sound is picked, the frequency indicator should be reliable.
It is at the end backed up by the internal quartz oscillator of the device.
However there are many possible bugs and design flaws that would make a tuner app inaccurate or not working at all.
Simply counting ups and downs per second may result inaccurate reading due distortions and background noises.
But if a good program can be written for a tuner, such a program will probably run on a smartphone also.
As a result, you probably need to try multiple applications and do not rely blindly on them.
If the familiar melody seems sounding out of tune, it probably is, even if the smartphone tuner suggest otherwise!
music.stackexchange.com/users/8737/h22
Smartphone tuner apps are pretty solid.
I have a specialized tuner that is 3-4 years old that has been having a hard time picking up acoustic sounds as of lately.
I bought a smartphone tuner app to tune my harpsichord and it worked like a charm.
I have experience with Cleartune for Android.
There are benefits to having a dedicated tuner.
For instance, if the instrument that you're tuning is electric, or acoustic-electric, you can plug it in to some tuners and tune it even if there is noise in your vicinity.
With the smartphone tuner, any additional background noise may get picked up and make it harder for you to tune your instrument.
music.stackexchange.com/users/9131/MrTheBard
A Tuner for Android 'Claims to be most accurate https://play.google.com/store/apps/details?id=jp.nokubi.nobapp.atuner.basic Auto & Manual Sensitivity Change accuracy levels Strobe rotation & in tune color I found that alot of the tuner apps actually have a 5 cent tolerance (meaning they say in tune but really not) a Guitar thats set up properly falls into a stable tune like a magnet & most app tuners
get you close enough the pitch goes in tune.
Another tuner I invite people to try out is Quantz Tuner / see your sounds
https://play.google.com/store/apps/details?id=com.QuantzTunerFree See your sound with this real-time, quantitative music visualization app.
After trying alot of tuners, I have noticed alot clamp onto a certain piont in the pitch and reads that result.
I think if your trying to set up an instrument or trying to adjust pitch competitions, might be better to purchase a real frequency tumer
music.stackexchange.com/users/43711/Android phone
I play the violin, and have always tuned using a smartphone app. Today I came across this specialised (to a certain extent)  electronic tuner , which got me wondering: How reliable are smartphone tuner apps, in the sense that why should I spend the money on a specialised tuner when an app seems to work just fine?
Are smartphone tuner apps reliable?
music.stackexchange.com
<tuning><violin>
music.stackexchange.com/questions/17347
music.stackexchange.com/users/10315/jon2512chua
Most people consider that smartphone tuner apps are fairly reliable. Although some people do believe that tuners, like the Korg orchestra tuner are better. The important thing is that the smartphone is able to pick up the sound.  Whilst some people would prefer a dedicated tuning device, it is generally accepted that a smartphone app will work just as well.
Some people prefer to buy a dedicated tuner. In general, it is considered that a smartphone app is likely to be good enough.