IMHO, Apps can look (and maybe even "feel") any way they want. But when it comes to integration into the system, they should be as native as possible. It'S 2025 and I can't drag and drop from/to the Gmail app on iOS - that's ridiculous (especially because I can't have real-time push notifications for IMAP in the mail app for Gmail account, which as far as I know is also a limitation by the Gmail servers, not by IMAP itself).