I don't know how this got this much negative reviews, but I'll tell you my experience.
I have been using
MailTag.io for more than two months now, and I am pretty much satisfied with its performance.
Unlike other email trackers, this one didn't slow down my gmail, and that is something that made me stick to it.
It comes with email scheduling, email tracking and automatic pings, and that has made my job a lot easier. They show a notification whenever an email is opened, and I've segmented the email addresses based on the activity - like time of opening, location, etc. Now I schedule emails so that they are sent when the subscribers are most likely to read my emails. Doing this has increased my email open rate by some 17%, which in turn has increased the sales.
No downsides so far. Good customer support, affordable pricing and works well. But yeah, it is limited to chrome and desktops.