Keep in mind that URIs are identifiers. They don't necessarily have to provide developers with the implementation details of the server-side.
From the API consumer point of view, the hierarchy or the relationship between user and comments should be explicit in the resources representantation, not in the resource identifiers.
If you consider the whole collection of comments to be a resource, then provide the resource with a URI, no matter how the resource is modeled in the server-side.
/comments is perfectly valid. And be quite because the API will continue to be REST.
I will share here some links that might help you to look at the problem from a different point of view.
(A bit old, but still valid)
Axiom: Opacity of URIs
The only thing you can use an identifier for is to refer to an object.
When you are not dereferencing, you should not look at the contents of
the URI string to gain other information.
For the bulk of Web use URIs are passed around without anyone looking
at their internal contents, the content of the string itself. This is
known as the opacity. Software should be made to treat URIs as
generally as possible, to allow the most reuse of existing or future
This axiom has not changed over the years.
Consumer point of view
1.2.3. Hierarchical Identifiers
The URI syntax is organized hierarchically, with components listed
in order of decreasing significance from left to right. For some
URI schemes, the visible hierarchy is limited to the scheme itself:
everything after the scheme component delimiter (":") is considered
opaque to URI processing. Other URI schemes make the hierarchy
explicit and visible to generic parsing algorithms.
Developer point of view
1.2. Design Considerations
A URI often has to be remembered by people, and it is easier for
people to remember a URI when it consists of meaningful or
While the URI is opaque to the consumer, it's not necessarily opaque to the human. So we still can make URIs readable as soon as it doesn't condition the system. Ultimatelly, the requirements has the final say.