For larger projects, product changes, legislation updates, Khan Academy makes it easy to transfer information in an easy-to-digest manner. The software works really well with adult learning as it keeps users engaged in quick bite-sized lessons. It's more appropriate for larger changes and bigger implementations. It's less appropriate for smaller projects, or minor changes/updates, where quick training sessions would suffice.
Quizlet works best with specific units where students need to know a good amount of vocabulary words, maybe between 20-50. It also works best when it is used both in and out of the classroom. It seems less appropriate when there are not many vocabulary words for the students to review.
I've only had to seek support one time. There was a glitch in one of my lessons and I submitted an email. The glitch was fixed by support within a few days, but I would've liked for it to be faster. However, I don't have a big complaint there because it is a free service.
The number one reason to choose Khan Academy over other options is price. If you are looking for a special skill or a quick answer YouTube is a great resource. Khan Academy is better suited for those who want to have a pseudo-classroom experience while still learning a subject or skill. I have used Lynda and Udemy; there is little difference between Khan Academy and those two options.
Quizlet is better because it allows students to go at their own pace and study independently. It is much easier to navigate as a teacher and as a student. Quizzizz can be more fun since it acts more like a game, but Quizlet is much more accessible for all learners with various modalities