Its a bit more complex than that and a lot has to do with the SoC manufacturer. The Armbian guys do stellar work and are.
Certain suppliers such as Allwinner are full of proprietary code and the problem is many updates are what enthusiasts have reverse engineered with many hitting a cul-de-sac where they can go no further.
The OrangePi & BananaPi boards are very true of what you say and like the Raspberry they actually suck when it comes to I/O.
The Olimex products are supposed to have good opensource that is current.
The Odroid stuff is supposed to be OK.
Rockchip apart from the MALI graphics are 100% opensource and they are pushing cleverly into that area.
The asus Tinker board uses an RK3288 which was there chromebit really.
With the RK 3328 I might end up wishing I had waited until the RK3399 Firefly has come down in price and maybe alternatives using that chip had arrived but I am expecting reasonable support.
So far there are quite a few images to choose from but mainly Debian Jessie / Stretch & Ubuntu Xenial with also various Android images to choose from.
Its a really new board but it has started with extremely good support.
Not really sure how well the Helios4 will be supported but the Rock64 is looking extremely good as you can see on the main page.