Love the minimalist approach - but not for business use. I use MixMax ( https://mixmax.com/ ) every day and the additional functionality its provided has been a big deal for me. More is more in a business use-case I think.
Rohit from CloudMagic here. To build these 'additional functionality' on all platform, you need to roll out your own clients. We've started with the table stakes, stay tuned for those power features, across all devices. Extensions don't work on mobile devices and 70% of emailing is done on mobile today.