the following query generates illegal SQL if the entity uses an embeddable as primary key:

select count(distinct o) from SomeObject o

This is just the simplest example for such a query. In reality the query is much more complicated and involves left joins (the reason for the distinct). It does not matter whether this is done in JPQL or as a criteria query.
The generated SQL is not legal in Oracle:

select count(distinct alias.field1, alias.field2) from SomeTable as alias

Trying to reformulate as e.g. distinct concat(field1,field2) fails in HQL. For Oracle in particular the correct SQL should probably look more like:

select count(*) from (select distinct f1,f2, .... from ...)