They acquired K-9 Mail a year ago or so, but it's still K-9 Mail. There's plans and a roadmap, but not much has happened that the end user can see, yet.
I've been looking at it recently because I'm currently using Mailbird. In recent weeks they told us that support for their current client will stop in 12 months time and we need to get on their latest client, which they want a one off payment AND an annual subscription. What greedy ducks.
Some points I remember from the time I ditched it (around 3-5 years ago). They may changed them:
The payment was linked to the version. Wanna get a new major version = pay for the new license. It's like buying the MS 20xx package but in that case you only have the single client instead of a whole suite. (Ignoring the whole price difference)
Client UI wasnt my cup of tea
Multi account didnt feel as comfortable as it does feel on outlook.
Had some issues connecting my Gmail accounts and keeping them connected. Not aware of any issues with outlook
Afaik the database went bad at some point. Maybe imisremember it though
In essence, most issues were personal. Try it for yourself. Maybe the newer version suits you more than me!
Update: Seems like I either misremember it or I bought a license because of a feature (at the time). Anyway: They appear to have changed the licensing for home use to be free and only corporate users need to pay.