Where is ProtonMail’s data stored? Where are its web servers? Who has physical access? Who has login keys/credentials to storage and server machines? Who does security audits, how are they done, when we’re they done last, what were the results, and what steps are you taking to improve your system’s security? And most importantly, what exactly does ProtonMail do when dealing with authorities and other entities that want access to user data?
Security is a process, not a destination - that’s a mantra everyone in the security world learns early on. But trust is also a process, not a destination. As an example of a company that treats both as a process, consider AgileBits, the developer of 1Password. Their white papers are case studies in transparency.