Hey,
In the rather nice series of books from Joshua Dazelle, 'Expansion Wars', there is several time a kind of justification that I don't get. Here it is.
Ships have a maximum speed they can reach, much much below the speed of light (so I'm not talking about the increase in mass by nearing C), past which the thrust of the engines is compensated by ... I don't know what! friction, gravity?
That said, the author seems rather knowledgeable of many physical processes, so I don't get why. If I'm not mistaken, if you apply constant acceleration to a starship/rocket/shuttle you can reach ultimately a speed very close to C. Even at 99% the speed of light, the increase of mass is relatively low.