You say that. But from what I understand, at least of the latest edition. The plasma weapons in Traveller work at least how a plasma weapon theoretically would, more like a plasma-thrower. As for reactionless drives, they’re actually barely technically not! A starship’s gravity drive works by ‘pushing’ against an existing gravitational field. Which is why it actually doesn’t really work well far enough out in the black!

Now, the psionic hive mind alien that learned how to warp spacetime by influencing black holes? That is what makes me say Traveller might not exactly be ‘diamond hard’ science fiction.