It's pretty easy to imagine fusion being great - but it's still just in our imaginations. No one has yet been able to build a working fusion power plant. There has been progress over the decades that people have tried, but its still a way to go yet. So although we can imagine that it could produce clean and plentiful energy, we just comparing sci-fi tech to current tech. The future reality might not be so great, and the current reality is that fusion power isn't possible at all.

To illustrate my point, lets imagine solar power from a 'theoretical' point of view, like fusion is described. Solar power uses no fuel; gets its power from sunlight. There is enough energy coming from the sun to meet the whole world's energy needs with just reality small amount of area. Solar power produces no waste biproducts... So if we just imagine the benefits of solar power, it sounds pretty much perfect. In in reality though, although solar is very good, it is still far from perfect. Construction, maintenance, and disposal of the panels are where the costs are. And so to compare to fusion, we'd need to know what it would take to build, maintain, and disposal of the fusion power plants. Currently we can't do it at all - so the costs are basically infinite. But even if our tech improves to the point where it is possible... it's hard to imagine it will be easy or cheap - especially because there will be radioactive waste. (Radioactive waste not from the fuel, but from the walls and shielding of the reactor, as it absorbs high-energy particles produced by the operation of the power plant.)