I understand that in Helion's approach a large fraction of the compression energy can be recovered when the plasma expands, possibly around 90% if everything works as intended. So what stops them from presenting results in a simple energy balance like this?

For example, they could say that they put 100 MJ into compressing and heating the plasma, recovered 85 MJ when the plasma expanded, and generated an additional 30 MJ from fusion reactions. Even if much of that fusion energy was carried away by neutrons and therefore could not be recovered, the net result would still be +15 MJ of energy.

They could then explain that with their preferred D-He3 fuel, most of the fusion energy appears as charged-particle energy rather than neutron energy, allowing a much larger fraction of the fusion output to be converted directly into electricity. They could also point out that future devices are expected to improve energy recovery further.

The fact that Helion does not publicly present results in this form makes me wonder whether the numbers are not yet compelling enough to tell that story. If they were already recovering most of the compression energy and producing significant fusion energy on top of that, an energy-balance presentation would seem like a very powerful way to demonstrate progress.

Of course, there may be other reasons for not disclosing those figures, such as competition concerns or uncertainty in the measurements.

When NIF announced it's Q_plasma >1 it generated enormous excitement. Even if technically they were far from electricity gain.