Beginners will most certainly will have highly inefficient start with it.
There are others in Ruby world which needs more praise such as Sequel and ROM(which is what Hanami uses also).
With Rails having the monopoly and most gems having AR as dependency, it's hard to see where we will go regarding ORMs in Ruby world.
However, that "boilerplate" makes data access and return values super clear. It also allows you to segregate Ecto queries by domain, meaning in a User context you may only have access to certain fields, whereas in a data processing context other fields may be available. This is great for coarse table access control, and you get it almost for free. It also allows you to very easily validate data at the edges of the system.
As for the query language, I can say for sure that there's a learning curve but it's close enough that your 1st guess is usually correct in my experience, and it removes a lot of foot guns like n+1 queries by default.
TL;DR there's some learning curve and extra LOC to write, but you get a lot in return.