1. Obsidian: has bidirectional linking and knowledge graphs, is faster and better supported, but doesn't have the outlining structure that Roam is based on.
2. Logseq: basically an open source Roam with 90-95% feature parity and some cool stuff that is its own.
3. Foam/Dendron/Athens etc: Roam clones.
4. Notion/Evernote/something else entirely.
Roam's about the block within an outline, interlinking and transclusion.
Dendron's core schtick is making traditional hierarchies easier to work with.
Like, with Roam a feelings of power scenario would be queries getting you subtrees from disparate outlines from which you pick out relevant trains of thought to synthesize your own writing, or just an outliner's ease in manipulating your text.
With Dendron it's more like, "oh, I've been thinking about this all wrong" and restructuring your notes so your new hierarchies reflect your new understanding.