What happens if you install another phone's kernel on yours? What would happen if you install a Oneplus 1 for an example on a Nexus 5? Would it cause hardware problems, like my touch won't be recognized or something like that?

And how much tweaking would it need? I don't know anything about kernels by the way.
At a simple level, the effects would be catastrophic. The system wouldn't boot, and you would probably hard-brick OR super-brick. Kernels are the direct system for managing communication between software and hardware, among other things, and the kernel needs to be just right. If the device hard-bricks, a USB jig would need to be used to force the device into a usable state, or just get a new phone. Super-bricks (which are usually Nexus-only, since even a Nexus can recover from a hard-brick easily) will require a new phone completely, whether you like it or not.

Luckily, it's a Nexus. It takes A LOT to break them. It will definitely soft-brick (the primary OS is unusable, and the rest is fine) and probably hard-brick (not too likely), and a very low chance to super-brick.

To sum it up..


No, kernels DO NOT work on other devices
You would be really, really, really lucky to have a usable device. By that, I mean that you would have to be lucky enough to get into recovery or fastboot and install a stock kernel. Then, plain sailing.