Flex Lessons is similar to these other platforms in that it exists as an online means of improving one's skill at the piano.
However, that is where the similarity ends. Flex Lessons is entirely focused on helping students become classically trained. There is a significant emphasis on the importance of learning and reading large amounts of sheet music. There is also an emphasis on standard piano technique, music theory, and other supporting skills.
Unlike many other options, Flex Lessons is very personal. The platform is designed such that students feel like they are individually supported by the instructor. Even if a student chooses not to participate in live lessons, students are still encouraged to submit recordings of their playing for feedback, which is absolutely essential for developing serious skill.
Lastly, Flex Lessons will not support an unlimited number of members, due to the interactive nature of this platform. Membership will be limited at a certain point.