>"the situation kind of reminds me the issues we have trying to achieve fusion energy, in both cases we know the that phisics work, we know is a desirable outcome but we need to workout the engineering problems"

Sometimes, engineering problems can themselves be insurmountable or simply not worth implementing. It's not a great feeling, but it's happened in the past and will happened again.

Nonetheless, the low payload capacities of SSTOs are a big risk because there may simply not be a worthwhile market for such small sizes. So many people focus on kilos to Low Earth Orbit while failing to take into account any other factor. It matters very little if you can get 1 kilogram to orbit for mere pennies at a time if no one wants to launch something that small, and a lot of payloads require additional space or higher orbits that any near-future SSTO would be hard pressed to accommodate.