Pine64 are pretty good for CPU and RAM, but you will be loosing out on the great support things like Raspberry Pi SBCs have
Edit: forgot to mention Pine64 starts at like 15 usd for 512Mb Ram or 19 usd for 1GB and like 29USd for 2Gb also from 19 usd up they have gigabit(can be hard to max out due to various issues IIRC, check forum), and the high end version is still cheaper than a RPi3 IIRC
Anecdote warning: As the happy owner of quite a few RPi, I had a really poor experience with the Rock64 (I know, it's not technically a Pine64). Multiple SD cards, mulitple power supplies and I can't get it to run for more than 5 minutes at a time without locking up. Doesn't appear to be a thermal issue as far as I can tell. Of course I probably just got a bad board, unfortunately after multiple emails I can't get a response from their customer service. I'm ready to just throw it in the trash. Really disppointed because I really just wanted an RPi with usb 3.0 and gigaibit ethernet, it seemed perfect.
On my Pine64 I use Dietpi from http://dietpi.com/ and it just works, it even has a setup gui where you can install to USB Drive and it will have bootloader on SD Card IIRC
I guess as long as they won't ship more than a couple GB of RAM they won't bother with it.