Porting to the ARM (64-bit) architecture was relatively easy (and needed for some other projects). Modifying the build system to handle bootloaders other than grub and distribution formats other than iso images and special per-device kernels? Easy to hack but not that easy if you want to do it right. We needed to do something that doesn't go quite as far as OpenEmbedded/Yocto, but still is powerful enough to handle many different devices. Making the hardware work? Largely solved by the upstream community. Finding or developing apps for base functionality, and making them work for the phone form factor? Mostly solved (with a few caveats). But how can we work with Android and/or iOS apps (currently required to e.g. log in to most bank accounts)? How can we make this appealing to a normal user?