In all fairness, you're underestimating the EPG. The EPG are likely some of the most dangerous combatants around that aren't force-sensitive, they knew several martial arts, and obviously had enormous proficiency with melee weapons.
Also, the EPG armor can resist lightsaber attacks (to a certain extent), blaster bolts, etc., via a mag-field that powers their armor. This mag-field also puts the Guards in constant pain and that forced pain management likely makes them even deadlier.
Addendum: Snoke knew that his EPG would face lightsaber wielders at some point so there is an implication they had familiarity dealing with force users and perhaps had some sort of protection? I'm unsure of this, though.
I think Sidious would take this but I also think Ren is underestimated a bit around these parts. I agree we don't have as much info on his capabilities, especially when compared to Sidious or Grievous, but I still think he's undervalued as a fighter nonetheless.