I purchased Letterly to use it on both my phone and PC (TIER2). Unfortunately, the problem started here as there is no traditional login system with a password and email. Instead, a magic password system is used, which is not optional and seems cumbersome.
Linking with a Google account is required, which I perceive as unnecessary surveillance. The 15-minute limit is also unnecessary and restrictive. 50 years of humanity with a classic password system instead of offering something else as an option is being curtailed without being asked.
Additionally, it is not possible to merge or further edit messages with additional content in a convenient way, which severely limits usability. Furthermore, audio files cannot be uploaded.
It is also not possible to give instructions on the fly via voice command, which unnecessarily prolongs the work. Overall a nice idea, unfortunately not thought through far enough.

Conclusion: While some users might be able to live with these limitations, my overall experience was negative. The software has potential but is severely limited by too many restrictions.