That doesn't strike me as being entirely true, unless I misunderstand your needs? Hubspot does this - my sales guys send emails via Google, I see everything in Hubspot. Prior to Hubspot we used Close.io - same thing. Every email sent (and every reply) showed up in Close.io in the context of the contact(s).

As for IMAP tags etc. - yes I've never seen that. Although you can probably use Streak for that (streak.com)