There is an interesting discussion of Machen on Spike magazine.
It says some complimentary things, but the conclusion is less favorable. To be honest I see the difference between Machen, James and Lovecraft to be generally a matter of taste really. They were all successful in a narrow form of literature. James's success owes much to the fact he takes fewest risks perhaps.
"In the end Machen is a clear link between James and Lovecraft, but in my view falls well short of either, lacking the simple clinical craft of the former and the sheer disturbing vision of the latter."
Link here: Arthur Machen