I never used an iMx8 which is still a future product that is not yet available widely. I have however used a bit the iMx6.

It would be difficult to compare it to snapdragons and other chipsets design for smartphone as it is not their primary target (as can be seen in their documentation : https://www.nxp.com/docs/en/fact-sheet/i.MX8M-FS.pdf). They don't integrate the baseband for example (which in this case is a selling point ).
iMx is a range from Freescale (then NXP, now Qualcomm...) targeted mainly for embedded systems, they cover industrial, automotive and consumer applications in their datasheets, but I have not heard of any smartphone using one before Purism. There is nothing preventing to use it for smartphone, but you have more luck finding one in your car or some handheld device.

What is great for embedded systems is that :
* you don't need to order them by thousand, they are available in usual distributors : https://www.digikey.com/product-deta...freescale_imx6 or in modules : https://www.toradex.com/computer-on-...freescale-imx6
* They have good linux support (Toradex provides yocto layers for them for example)
* They have OpenGL capable GPU, great for fluid user interfaces
* Extended temperature range are available
* they are guaranteed to be produced for 10 to 15 years (smartphone chips change every year...)

So they are great and powerful in embedded systems. iMx6 are still on the older Cortex-A9 architecture (one to 4 cores), and iMx8 should be a lot more powerful with newer generation 4 cores Cortex-A53 (ARM-V8 64 bits, against ARM-V7 32 bits).

Here is the kind of thing you can do with them, demonstrated by Qt guys themselves : A 3D animated multi-screen car dashboard : http://blog.qt.io/blog/2016/03/03/cr...sters-with-qt/
By the way, it is a platform really well supported by Qt (so KDE plasma should be a breeze to port on it), as it is part of Qt's Device Creation offering : http://doc.qt.io/QtForDeviceCreation...platforms.html

If you compare that to the high frequency octo-cores found in Android flagships (like galaxy S8), it can be found weak, but it is the same architecture as found in the Odroid-C2 that powers a lot a Kodi 4k players without any lag...
It would be a lot faster than my Jolla 1 which I don't find lagging in daily usage.

And regarding the mainline kernel objective of Purism, it looks like a really good choice, seing how the etnaviv (open-source driver of the vivante GPU) has been integrated in the 4.8 kernel, and Mesa supports it.

About battery life, I wouldn't expect it to be worth than on android flagship, due to these chip being designed to be embedded : "Optimized for fanless operation, low thermal system cost and long battery life". We'll see, I have no hard data to compare on that.

I don't know if this answer your questions, If not, feel free to ask for more details.