There are similarities between Summoning, Ulver, and Agalloch.
All three have (fewer or more) elements of black metal. All three's tales take place in environments like the depths of a forest, or atop the heights of mountains.
But there are also differences, some of which make Summoning a superior metal band.
Ulver is passive and docile in the presence of magnificent nature. Summoning is invigorated in its presence.
Agalloch has a woe is me attitude and dwells on petty issues with a small minded, egocentric view, giving the impression that the imagery of desolate winter is merely used as a metaphor for an individual's state of mind.
Summoning sees humanity as a part of the greater nature, and thus, avoids that view in favour of a grander one.
My advice to like minded people: Listen to Summoning. Don't waste too much time and effort on Agalloch. Don't make the mistake I did.