As is every other Android device. The situation has gotten better with new SOCs though, so it doesn’t apply as strongly to new devices.
Fairphone doesn’t have the money to achieve this. With huge amounts of money and huge amounts of time it would be possible there are few advantages (for running Android)
Every OS on the Fairphone 2 is using this 3.4 kernel.
I’ve been using LineageOS 16.0 for about 1.5 weeks now; so no, no (direct) problems because of the kernel. Thankfully Android doesn’t really care what kernel you are running (if it supports the necessary kernel features - which can and were backported to the 3.4 kernel)