I have been a loyal user of Noteflight for the past few years (since it was the first notation software I was introduced to), but I have always wondered why it did not appear on the top of the list, losing to apps like Sibelius or Musescore.
I tried out Sibelius, but found the UX too confusing and overwhelming. Also, the host app, Avid, was too memory and compute-intensive and was quite an annoying software to use.
However, I recently began to self-host Musescore, and I found it quite nice. Musescore is also open-source (as supposed to Noteflight and Sibelius, which epitomize proprietary notation software).
Noteflight has been recently acquired by the Muse Group, Musescore's parent company, and that has probably reduced the need for improvement upon Noteflight's notation editor features. I believe that Muse Group (and Hal Leonard before it) is using the softwares for different purposes: Noteflight is supposed to be a collaborative, educational app, with an easy learning curve, whereas Musescore is meant to be the professional app.
However, I believe that MuseScore is almost as easy to learn as Noteflight - they both have virtually the same UX features and UI designs as each other, at least for basic features. Compounding on that, Noteflight is limited in storage, and is very slow for processing. I would think that people should stick with Musescore for educational and professional purposes. Noteflight only seems to have an advantage on a collaborative perspective.