We do know there are inherent problems with fusion, particularly DT fusion. In fact, we've known for 35 years or more. Even if the plasma physics issues are magically solved, fusion has grave engineering obstacles.

http://www.askmar.com/Robert%20Bussard/The%20Trouble%20With%...

https://pure.mpg.de/rest/items/item_2131865/component/file_2...

It's interesting examining the recent efforts, public and private, in light of these venerable critiques.