Lore means SM are pretty badass but it really depends of the writer and story
Like if the novel/fluff is about SM then they chuck norris in power amors.
If the novel/fluff is about IG they are usually never seen or IG leader can battle easily with Chaos SM or gain respect from Imperial marines (like in Gaunt novels)
If the novel/fluff is about SM enemies then they have butter armor and pansies guns (Storm of Iron etc, some short stories.)
If you go the "fair" way SM are indeed pretty badass and superhuman but far from invincible, especially the "basic" tactical marine. Lots of creature/badass/weapon in WH40k universe that can crunch a SM in pulp.
For a random example a "basic" Chaos Marine from one of the original Legions is supposed to be:
A SM
PLUS 10,000 years experience (compared to the 100-300 years in average of a imperial SM)
PLUS Chaos Powers (mutation, spells, special demon-weapons etc.)
MINUS Faith to the Emperor (And They Shall Know No Fear)
MINUS Some of the new tech Imperial SM have (plasma weapons, stormbotler, razorbacks etc.)
PLUS Some of the old tech the Imperial SM can't have anymore since they lost it in the past 10K years (some genetic advancement or armor gadgets)
I would say it's more than an equal match... or in a game like Space Marine a Chaos SM will most probably become fodder... and a single marine will easily beat 10 of them at the same time
Though personally I will greatly enjoy the game for sure, Coop or not but it's hard to say how powerful SM "should" be