Cons of using a star schema in a columnar database
You are mixing up three orthogonal concepts: data model (star schema), workload characteristics (OLTP vs. OLAP), and physical data organisation (columnar).

Your data model has no bearing on whether column-organised tables are appropriate for you; however, data organisation must reflect the nature of your workload (i.e. queries). For example, SELECT * FROM... cannot possibly perform well in a columnar database.