Do you blog on Dev(.)to or Hashnode?

Neither. I use Github Pages (I'd also recommend Netlify) and host your own.

I wrote an article on each to see if a backlink would affect my domain authority. It didn't seem to.

I have both, currently linked my domain to Hashnode blog https://ideaman.io . Its very easy to link your domain. The only thing I hate about Hashnode is their like buttons (with medium you have one like/clap button) with Hashnode user get 20 options to express their support, that looks ugly to me.

I haven't looked at Hashnode beyond scanning the landing page, yet, but I'd classify Dev as more of an open source social network that features blogs, rather than a blogging platform per se. They've given a decent amount of thought to on-boarding, and they seem to do a good job of acknowledging that a lot of developers are going to already have a blog and want to just get their content to a new audience, in their case through RSS and links back home. They also have some nice custom templating commands for internal and repository hyperlinks.

I haven't been writing enough technical posts to push much their way, but I've been happy giving the site a couple of minutes every morning and yanking the occasional programming article off my blog for re-use there.

Now giving Hashnode a couple of minutes without signing up, it looks much more like just a straightforward blogging platform. The red flag (to me) is the comments. Every platform obviously has a spam problem (and Dev had a rough week with spam posts), but the rare comments I saw on Hashnode were either (paraphrasing, of course) "you clearly just hate (language/platform/framework/library)" or "I enjoyed learning a lot about (insert post title...or don't, if the script failed)."

So, I probably wouldn't like it, there, since I already have a blog that I'm happy with (and can get away with posting about what I'm reading), but that doesn't mean that you won't like it.

I'm doing content syndication on both, Dev.to and Hashnode with my blog: https://www.indiehackers.com/product/box-piper, https://boxpiper.com/
Dev.to proves to be more worthy and Hashnode is not providing enough traction right now.
In my opinion, Hashnode works brilliantly for those who already has an existent followers and take lot of time to build community.

Don't have any experience with Hashnode, but can highly recommend dev.to. It's really easy to cross-post content from your personal blog and community is very nice and engaging. Feel free to follow me there as I'm planning to blog more regularly soon: https://dev.to/clarity89.