I don't feel like i've had the same experience as this,  several times within missions (doing power level 4 stuff) i've seen CSM take me down in 1.5 seconds due to a mere bolter gun. They also take 5x longer to kill than any opponent when the party all jumps on-top of them. In truth I think they are a pain in the backside.

Regarding their threat level I see it as similar to the table-top - If you have one marine with a bolter shooting at you in addition to a group of cultists with autoguns/cannons - Then of course the CSM is going to be less of a threat, its still only 1 gun that they have  in their hands after all and limited damage output.

Perhaps some minor tweaks to their health etc, or an increase in base movement speed would make them more scary.. But if they were any any tougher then I think the harder difficulty settings (fighting against higher power levels in group play) would become impossible.  

If you are level 3 then anything 3 or under is supposed to be easy, that's the point in the power level system! 

I think if anything it's an issue of gear scaling too quickly rather than enemies being too easy. But when comparing the stats of a normal unit to a csm there feels something like a 4-5x difference in health, which to me at least feels about right during group play. Considering there will also be chaos champions and other units added at a later date I don't think a CSM should really = an elite or a boss, but I appreciate the argument for why people might want them to be. 

Thats my entire point :D level 3 vs level 3 things need to be "fairly" easy - This is 2017, unless a game caters to "casual players" (not meant to sound offensive) then it'll just come under fire. 

For most of us who want a challenge I think we are expected to play against +1/+2/+3 power levels, just like a difficulty modifier. And when +3 in power level I find those CSM are a B##ch to deal with :D (as an assassin anyway)