Yeah, s-exprs don't break down by lines very well, but they are easy-ish to work with in a structured format, similar to HTML/XML editors since the delineation between elements is usually very clear. The only problem with lisps would be when macros are involved where there is more fluidity in the structure. But something like this:

  (defun double-all-the-things (things)
    (mapcar (lambda (x) (* 2 x)) things))

would be straightforward to navigate through with an s-expr aware editor.

It's more that the selection operates on whole lines in Tree mode, so it would parse fine with the tree-sitter grammar (for syntax highlighting etc) but selecting the inner contents of a block would take the block's closing ) with it. This would only matter if you wanted to move the contents somewhere though - editing and refactoring commands (including with the help of LSPs) will work, with the selection being the outermost node on the first selected line.