15.1 is weekly again …
Source?
The power button works.
Perhaps you are referring to the bug with the phone rebooting instead of shutting down? This had a simple workaround (activate the flashlight before shutting down), and is fixed in the current Fairphone OS and Fairphone Open OS 19.02.1.
The fix has yet to be implemented in LineageOS, but … simple workaround.
Or the bug with the phone not powering up while charging? This was fixed, too, and the fix can be applied to a LineageOS installation already.
Or the one with the power button tending to get stuck, which you could easily repair yourself by swapping it with the camera button … or get the cover replaced for free by Fairphone? That was ages ago.
And WiFi trouble? In general, WiFi works.
Sure, some users have WiFi trouble, and this forum is where they report it and perhaps get any help.
But before you even tried your phone, don’t jinx it 
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Oh, please do, under all circumstances 
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Depends on what exactly “onboard” is supposed to mean and what scope of data you want to copy.
Often mentioned here in the forum are Titanium Backup and MyPhoneExplorer (Windows only if you want to connect via Bluetooth or USB).
How to install LineageOS on the Fairphone 2 is here …