Frontiers has many different journals. The quality of the journal highly depends on the editor of the journal. But there has been reports that Frontiers has been involved in predatory actions. They used rejected papers and re-submitted them to other journals (as they have a lot of journals and some paper topics can fit in several journals). This is seen as predatory behavior. Overall I would argue that Frontiers is borderline, it would not be my first choice. Keep in mind, as an Open access journal they get paid my the author's. Each article costs around 2.000$, so they need to publish papers in order to survive as a business.