Mostly their marketing practices. They are designed well but mostly designed to keep you locked in one way or another.

For me, their desktop is not as intuitive as people make it seem and lacks simple shortcuts that most other desktops have.

On mobile, its the restriction of customization and options. They are getting better at customizing but still limit you on options for anything outside of their apps. They claim to be private but follow similar practices as other companies, just in a more quite way with better PR.