Khan Academy is a great supplement for things you already have learned, or as a mid-level course (versus a deep dive or high school/university level course). It is great for a paced learning environment if you want to casually refresh yourself on a subject or learn something new. I don't think this should be a replacement for structured learning, or if you struggle in self-paced learning environments. There have been times I would like to have been able to ask a professional (like an instructor) when struggling with a subtopic, which is possible in a classroom setting.
Quizlet works extremely well for students of all ages to study not only vocabulary but all types of academics. I have used it not only as a teacher to provide for my students, but I use Quizlet myself when studying or learning something new. The ease of access with it being online, at your phone, or wherever you go highly surpasses traditional flashcards.
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.
I personally found Khan Academy much better suited for adult learning of difficult or larger topics. Not so much for smaller changes that can be communicated via email. But it was more engaging and I found the information easier to digest. Change management can be tough but with the right tools it can be a breeze.
We used the free version and it proved to be a boon
Our interns learned about the data structures and algorithmic concepts.
We also use math-heavy calculations to derive things like statistics in our machine learning models. So Khan Academy helped us get on track for the statistics courses.