If you're referring to SQLa Core, it's still SQL. The AST is the same, but by building the SQL AST explicitly you prevent injection and get a degree of composability (with subqueries). IMHO it's the more maintainable way of writing SQL.
subqueries are best expressed in sql directly. They can get complicated, especially when using the with clause.
I am not saying that all of what ORM offers is bad. But beside the very basics they should step away. The less they offer the better.
# a subquery
demo_accounts = db.query(Account.id).join(Client).filter(Client.name=='Demo')
# used inside a query
print(db.query(Account.name).filter(Account.id.in_(demo_accounts)))
SELECT account.name AS account_name
FROM account
WHERE account.id IN (SELECT account.id AS account_id
FROM account JOIN client ON client.id = account.client_id
WHERE client.name = :name_1)
# or as a cte
da_as_cte = demo_accounts.cte()
print(db.query(Account.name).join(da_as_cte, da_as_cte.c.id==Account.id))
WITH anon_1 AS
(SELECT account.id AS id
FROM account JOIN client ON client.id = account.client_id
WHERE client.name = :name_1)
SELECT account.name AS account_name
FROM account JOIN anon_1 ON anon_1.id = account.id