Outranging things doesn't do much good when the gameplay requires you to get in close at a base to capture a point (hint: where there is lots of cover).
MRM's are a problem, but the mechs who have MRM's are generally going to die once the Demo gets in range (which isn't hard when their range is 650). And on any map that offers any sort of cover as a means to get in close (read: most of them) then it's pretty simple to evade fire.
The only real weakness they have is stuff like Gauss Rifles used at max range (which is only applicable in maps like death valley or if a demo tries to cross a bridge in Kagoshima that has heavy enemy activity on the other side).
In which case, the team switches to using Gauss Demo's. Sure... maybe the novacat with 3 ERPPC's, if he can keep his max range, will be victorous in this. In that case, the Demo simply needs to shred the mech's arms that carry this, then the novacat at long range is no longer a threat.
The only other options would be stuff like LRM's or Lt. Gauss... but these do so little damage (considering LRM's hit the top which is strongest, and they have LAMS) that the Demo will easily be able to close the distance.
Couple a Demo with a Morrigu that has 2 LAMs and long range capability, and you will steam roll everything. If you have a tank column, the only thing that can stop you is a bigger tank column or lots of shivas (if they don't have AA..).
Considering I can EASILY go toe to toe with a Mad Cat MKII or an atlas at half the price, it seems a bit rediculous.