I think, though this is a guess after only one session and some theory crafting, that a phased solo like the Volcanic Dragon can fit the bill as a legit solo threat. But normal solos are more like 4e elites.
Yup, this is what I’ve been saying and my experience yesterday bore it out!
The ones with relentless can be a good thematic “oh naughty word” challenge but not really a full boss. Which is fine! Sometimes you don’t want a near death encounter but do want a solo guardian or stalker or something. I think the “challenge” for the encounter math should probably have been a little better explained.