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First, 14.04 should boot but it will be largely unusable since it doesn't have the kernel and drivers for a NUC5i5.
So you want to focus on Ubuntu 15.04 which does.
I have never seen this problem despite booting from USB devices many times with Xubuntu, Lubuntu and others.
If possible, get a different USB drive, completely reformat it, and then add 15.04.
unetbootin is generally considered dependable but I've always used YUMI with my NUC if you want to try that instead.