The "power level" of the average astartes is one of the most inconsistent details in 40k.
Gameplaywise on the tabletop, one space marine is worth a bunch of guardsmen. Okay, makes sense, since you don't want to have an army of only like 10 models (like custodes).
In the lore, they range from anywhere between being just a tall dude in power armor, to invincible demi-gods capable of conquering entire worlds by themselves.
But let's be logical about this. Setting aside how many John Wick skills an Astartes has, their standard equipment is: power armor and bolter.
How strong is power armor? Very strong. But still weaker than most vehicles, both in gameplay and in lore. Same with the bolter, your standard Rhino with twinlinked bolter already outguns the average space marine.
Any of the Tau battlesuits should, on the face of armor and weapons, be more than a match for a single astartes. Forget the source, but I remember Crisis suits being compared to terminator armor, except lighter and basically better in every way.
So how much of a resource cost is a battlesuit compared to the training and outfitting of a space marine? Presumably, much much cheaper.
So while the average firewarrior may have trouble downing a space marine, their mass produced gundam knockoffs definitely can. Astartes are not great in cost efficiency.
Their elite, grimdark badass meme status should not be taken at face value. It's like how their bolters don't have sights on them, the original bolters looking like they were made by somebody with no gun knowledge, and using something as impractical as a chainsword.
It's part of the satire.
