I'll open that as a bug.
The Comments property is a 'text' field in the database (called 'Memo' by the Custom Property Editor). This allows for nearly unlimited size data (like 2GB), but it can't be compared, sorted, or grouped in SQL. The old non-AJAX tree works by fetching all of the nodes from the database and then grouping them. Since the grouping happens in application code, the database limitations on 'text' fields don't cause any problems. However, fetching all of the nodes on every page load causes performance problems.
So the AJAX tree doesn't fetch all the nodes. Instead, it uses SELECT DISTINCT queries to get all of the values for the groups. This is much faster, but it doesn't work with 'text' fields.
I would definitely consider it a bug that we took away the ability to group by a Comments property in the old non-ajax tree. But the nature of the ajax tree doesn't really allow us to group by Comments in that context.
As an alternative, you could switch to using a large-ish 'char' database field (somewhat confusingly called 'Text' by Custom Property Editor) for your Comments. This can be sorted and grouped anywhere.