My two complaints about Onsen are:

1. They are gender separated. So when travelling as a straight couple or in groups you likely can’t share the experience together. Some see this as a plus. I see it as a minus. Put it down to personal preference.

2. Must be nude. Ok well I guess I’m just a prude, but I don’t enjoy the concept of being nude in front of others. Rules out German style spas too.

3. Bonus issue. Tattoos. Because of the criminal association with Tats in Japan you can possibly find it hard to use an Onsen (Ie banned).