Why I switched back to Thunderbird (bugs/features of Geary)
This is more like bugs/features comment into one and it is a constructive criticism. Geary is great because it is super simple and looks great. It is modern. So I love it. However for "powerful users" it lacks key features and doesn't deal very well with spam. So, if you have many email addresses and you are receiving lots of emails Geary is not going to help you as much as Thunderbird.
In TB when I add 10 email accounts I can select for all the inbox, spam, or draft folders to merge. This is crucial, else you end up managing 10 different email addresses. To have all emails from all addresses merge into one Inbox is extremely useful. Geary does not have this feature but I've read that they will work on it.
TB gives you way more control over spam messages. Their "junk" filter quickly learns what is spam and what is not. With Geary I am receiving 10+ spam emails a day and I don't have any way to tweak the spam filtering (if there is any). So I end up managing 10 email accounts separately (since no unifying inbox) and in the same time receive lots of spam for each account.
TB let's you reply with any email account you desire. Very useful.
I am getting lots of emails so I want to reply to some later on. In TB is as easy as open them in new tabs and leave the tabs open until I get back to those emails. Works like a browser, something we are all familiar with. For now there is no such thing in Geary and to be able to do a similar thing I would have to manually add a label to the messages I want to get back later on, then search for that label. Not fast or intuitive at all. But the dev will work, as I understand, on something for Geary to be able to pin a label to the sidebar and then you can easily add those emails to that pinned folder - I would say it is easier in TB to open messages in new tabs, but maybe that's just me.
Of course I think Geary is geared towards users who don't want a lot of options and just want a great functioning email client. I think you are doing a fantastic job and I will keep an eye on the project to see if the above features will be added. Thank you so much for your work!