I have a similar issue with Matebook X Pro (3000x2000) and external 1440p display. Weyland scaling factor not apply to all programs as above.
I also checked other distros and desktop environments, it works somehow on KDE Plasma - I mean scaling was proper on second display - but there was a lot of other issues, for example, some programs not scale in general.
On Ubuntu I also notice touchscreen issue, it works, but it is not the best user experience for me.
I type it with pain, but... for now - it is really hard to write - for now only Windows 10 works fine with this setup. It looks like we have to wait for better Linux support of HiDPI.
Update
After about one week i gave up... Windows 10 is so useless. I am very impressed that OS provided with hardware works so bad (minimum 1 crash/hang per day, about 10Gig RAM used by OS, Candy Crash in menu, preinstalled, in profesional OS!! and many many more bad experiences).
Then I switch back to only right OS - Linux. I'm really sorry that I doubted him.
In some way I workaround this issue. I disabled scaling on both screen (1x scale factor was seted). Then on my HiDPI screen I changed resolution to 1920x1280. I know that is strange resolution but it is because my screen aspect ratio is 3:2. This resolution is apropriate to properly see UI elements and do not casue strange graphical glitches. Everything looks perfect. All programs works like charm. I was afraid when I changed resolution to smaller one then everything will be looking odd. I am impressed how everything works so great and there is no need for scaling.
I setted this weird resolution using xrandr (google is your friend).
For every day use my setup is 3000x2000, 1920x1080, 1920x1080 screens and finaly I can use this potential with power of only right OS.
Currently I am using Ubuntu but this should work on any Linux based OS.
Best Answer
You should restate your problem as a question since it's not clear what you expect.
A patch was applied to Unity 7 for the Zesty release that claims to improve automatic scaling on HiDPI displays, but it completely broke HiDPI settings and behavior for me as well. Like you, in Yakkety and earlier releases, I had Unity's display scale factor set to 1.5, and the UI and font scaling was perfect on my 2560x1440 display. After upgrading and rebooting into Zesty my fonts shrank to tiny proportions, and now I have to settle for a scale factor of 1.38 (which is almost below the comfort threshold for my old eyes) because at 1.5 fonts appear much smaller than they do at 1.38, which is obviously broken behavior.
Here's the changelog documenting the offending change in Unity:
https://launchpad.net/ubuntu/+source/unity/7.5.0+17.04.20161130-0ubuntu1
Here's the diff between the prior Unity release and the current, it contains the new (broken) scaling code:
https://launchpadlibrarian.net/295698352/unity_7.5.0+17.04.20161111.1-0ubuntu1_7.5.0+17.04.20161130-0ubuntu1.diff.gz