I just did a quick test of a pretty standard use-case for me, which is to search for a package's tracking number in my email. I literally opened the email in the Proton Mail desktop client, copied the tracking number, pasted it into the search box, and...no emails found.
Pasted the same tracking number into Apple Mail (which uses Bridge) and it found the email immediately.
Yes I have "Search message content" turned on and yes it finished downloading all of the emails.
So either the search function itself is fundamentally broken, or it's not actually downloading all of the emails to search them even though it indicates that it does.
This is just one simple example, but in general regardless of whether I have "Search message content" enabled or not, I have to pretty much know the sender's email address to find anything reliably.
It's really a major usability issue, and the main reason I don't recommend Proton Mail to my friends and family.