doesn't return search results for emails just a month or two ago. very very frustrating limitation while traveling and trying to pull up booking emails I thought would be instantly available to me when arriving to destinations.

Obviously Proton is an established as a leader in the privacy space. But honestly the mail app is a bit frustrating.
The app lacks a certain responsiveness and intuitiveness that has become a standard these days. If you're used to apps like Spark Mail, or even Gmail/Apple Mail, you may get frustrated. You will get your privacy back with Proton but will be left wanting more when using it day-to-day.
P.S. The search functionality is also needs work