The problem is that we need it to just barely break even and then a little bit more. Having a tremendously energetic fusion device is both useless and a solved problem: we already know how to light one off, but it only lasts a millisecond and it destroys everything for miles around.
For what it is worth, fission also suffers this problem. A promptly critical fission reaction is possible, indeed quite easy to achieve, and very undesirable. All fission technology is dedicated to keeping the reaction just barely critical.
For what it is worth, fission also suffers this problem. A promptly critical fission reaction is possible, indeed quite easy to achieve, and very undesirable. All fission technology is dedicated to keeping the reaction just barely critical.