I was doing a deep dive on this. I think fuel is going to be a limiting factor and production of it. Essentially a fusion engine just fuses hydrogen into helium. You need Deuterium and Tritium and fuel for this (hydrogen with one neutron and hydrogen with two neutrons). Producing a small amount for a test is one thing, but to produce this fuel at scale for a reactor to power a city is a problem. Also the actual energy generation is an issue. They used to use Beryllium as a neutron moderator to generate heat, but they have a new method now.