Udacity Likes: I loved Udacity's production quality & instructors. I think they are generally awesome in the balance of teaching concept and practical knowledge. They also don't shy away from more complicated topics like autonomous vehicles, InfoSec, AI / ML, etc., which is a unique asset.
Udacity Dislikes: They are placing too large of a bet on the nanodegree concept. There isn't a clear ROI for students - and it's fairly clear that most companies don't really give a hoot about online certifications.
Their classes are too expensive. Both Udacity and Udemy have great Web Dev classes. Udemy's was $10 for 19 hours of content + responses to any questions I had within hours, and gave me everything I wanted + more. Unfortunately, I had to sift through the Bootcamp and Nanodegree madness on Udacity.
They are unaccredited. My company (maybe most companies?) only subsidizes classes that are accredited. I have a $10,000 annual credit and I use it all on university classes. I would definitely consider Udacity nanowhatevers if I could use my credit on them.