Yes to the colonialism obviously, Boudica et al. But after that they did sustain the empire in relatively civilised way, for the times at least. I picked this up from Mary Beard. I'm not seeking to defend colonialism but it's one way the managed to hang on to it.
Constitutio Antoniniana - Wikipedia
"Citizens" didn't include slaves, women and a few other categories. Roman slavery wasn't as absolute as the British/American model and was transmutable - they weren't seen as an inferior class, more like indentured servants.