Personally I haven’t read in builds that have used it that it does offer a clear improvement.Perhaps someone who has used it will offer before and after experience.
Cons,
Does it actually significantly improve machining performance?Could be expensive for large quantities.Will make structure much heavier.Cant be removed.
Vibration reduction (or improved damping) can be achieved in a number of ways.Improve stiffness and/or increase mass.Or use a material with better intrinsic damping properties.
Steel is probably bottom of the ladder(EG,cast iron,aluminium,steel)in intrinsic damping but its stiffness/$ can overturn this disadvantage.
Best of those choices IMO is probably to increase the amount of steel used.This will add stiffness and mass ,efficiently reducing vibration.Closing the ends of tubes will add around 30% stiffness.Chosing a stiffer closed loop structure like a portal before an open loop like c-frame or moving gantry is best.
My choice of fill would actually be to half fill tubes with either river sand(cleaned) or steel (or lead shot) and oil.The advantage is its removable.
I would expect that the material carrying most of the machines loads will dominate vibration properties.Steel in this case.
I recall reading a report where a major manufacturer back filled a large cast iron slant bed lathe with EG.They saw a 30% improvement in damping,from memory.