Regardless of whether I initially liked it or not, for me Unibox is absolutely compelling to use. At first I was turned off by its people-centric way of arranging email by contacts (and I still don't care for that...but see the second paragraph). This concept probably works just fine for most casual users of email because it's more like chat or instant messaging, and in that regard it's actually better if you'll just give yourself time to get used to it.
For business and professional email users like me, this concept most likely won't work because we get a lot of daily email and most of it needs to be acted on and then either labeled (Gmail) or put into folders (other email types). Fortunately, Unibox has recently recognized the existence of this email user demographic and made preference changes that allow Unibox to function more like a classic email client. (The most important preference change to help this type of user is found here: https://www.uniboxapp.com/faq/organizing-emails/inbox-zero)
What will eventually win you over to Unibox (despite its high price tag of $17.99) is its absolutely beautiful user interface. The developer has uniquely managed to put everything into one single window, even the message creation window when called up. So writing a new email or replying to an existing email, allows you to focus on the email you're creating instead of bouncing around between multiple windows. No other email client does this.
Last, but not least...email providers. Very few email apps support all email providers, but Unibox "automatically" found all of my email addresses (Google, iCloud, Outlook.com, Yahoo, Hover Mail, personal IMAP server, and even Exchange (IMAP enabled). It needed a little help configuring my DOVECOT IMAP server account, but only to the extent I had to add the specific IMAP and SMTP addresses as the account is intentionally configured a bit strange in that regard.
Is Unibox perfect? Of course not...nor is any email app. But it's as close to perfect (for me) as I've found and it has become my email app of choice because it's easy to look at, efficient, font selections are uniform between inbox and messages, and it supports all my email addresses. Add to this the developer is very responsive to email support questions (a rarity these days) and I think Unibox is certainly headed in the right direction. The Apple ecosystem is filled with email apps, but Unibox is a true breath of fresh air.