It doesn’t really matter what kind of engine it is if it’s going fast enough.
Anything with enough mass and acceleration to move a human being from planet to planet in a reasonable timeframe has the kinetic energy required to wipe out a city. Once you start reaching relativistic speeds, you can take out entire planets by simply not slowing down on approach.
The fact is that any manned vehicle capable of interplanetary travel is by the nature of the energies involved, also a weapon of mass destruction. A spaceship is a weapon in the same way a car can be a weapon.
So either you massively restrict access to this technology, or you create a system of surveillance and defense that is so pervasive and effective that it makes 1984 look benign, OR you just say fuck everyone else and use that weapon to remove yourself from range of everybody else’s weapons.
Proliferation is an existential problem for anyone in range.
millions dead
https://military-history.fandom.com/wiki/Relativistic_kill_vehicle