THE GOOD:
Their FAQ says it uses OAuth to connect to your Google account and the emails never leave your browser.
It also costs 83 cents for 1 month so you can go nuts. Alternatively you can pay 1 cent per email if you have a handful.
I see another HN thread about it here from a few years ago: https://news.ycombinator.com/item?id=32462878
THE BAD:
I don't know if it can be used with total confidence because that HN thread makes it sound like they delete the original email and create a new email with the same meta data but some of the comments indicate that's not quite the same as the original. If you had to present this email as evidence, you could run into friction since the original no longer exists. Technically it looks like you can back the original up but as someone mentioned in a comment that seems like it would add a lot of friction in a legal case.
I have no intent on ever needing to use old emails with large attachments in a legal case but knowing this could be a problem makes me hesitant. Although on the bright side, it would be fine to use for anything you 100% know won't ever be used legally.