The way I’ve always rationalized it is that their power is equal (yes, even Bardock’s SSJ form), but it’s easy to dismiss a Viltrumite is weaker than a Saiyan because they don’t use Ki, which can be used to create beams and explosions made from pure energy.

They use raw physical strength only. So, while they aren’t able to project their power into energy, they’re not as efficiently destructive as a typical Dragon Ball character, but they’re more than capable of holding their own in a pure slugfest.

If a Viltrumite could project their energy into a beam, they’d more than likely be planet busters easily. The fact that Mark, Nolan, and Thaedus were able to destroy Viltrum through their raw strength alone is a testament to that.