NGAD has a cost issue. Till now no tech issues have come up. As far as we know.
I wouldn't be so confident about saying what the actual extent of the "issues" NGAD has are.
While cost is certainly a major element that is disclosed, cost is also very easily related with technology, requirements or capability (in fact, they're arguably all connected with one another)
Probably a good idea to not rule anything out.
However, what we do know is that either way the USAF over the last year seems to be reevaluating just what the actually want out of their NGAD effort.