It's currently buggy and feature lacking, as with most new applications. I would suggest giving this one some time to mature and improve in quality before purchasing it. Judging by Boxy 2 it has good potential for the future.
I've been using Boxy since Boxy for Inbox. As soon as they released Boxy Suite I bought a lifetime licence. I'm a huga fan and really recommend this product to anyone who frequently use the Google products they cover. The only complaint is the coloring of the dark mode theme actually. But you can change most of that in Gmail so it's fine for now :)
Boxy Gmail has bugs/"features" that interrupt important native functionality, and has visual artifacts that are bugs. Boxy Dash ignores preferences, and takes 20+ minutes to notify of new email, making it unhelpful for some of my emails. Shift works better. I regret paying for Boxy. My guess is they jumped to building their other products, like Unreadit, and have let some bugs languish that really hurt the productivity of Boxy Gmail. I've reported issues for awhile, but waited too long to ask for a prorated refund. They said they were willing, but after 90 days their payment processor wouldn't do it. But Boxy team declined to Paypal me, as well, with little explanation. "We can't do that, I am sorry."