Part of the reason D-T fusion is the primary candidate is that it occurs at temperatures and pressures orders of magnitude lower than He-3 based fusions. This makes Helions approach more difficult from a materials, heating, and magnetic field perspective.

I do research tangential research and one thing I've heard (take with a grain of salt) is that they don't publish nearly as many results as other fusion companies. This may not mean anything, or it could mean they don't have favorable results thus far.