Made hangouts handle my SMS. Any downside to disabling Messanging? If I've made Hangouts handle my SMS, is there any downside to disabling the default Messaging app?

Does anything rely on that app being enabled?
On some devices, especially Motorola ones, disabling the stock Messaging app will cause any app that you set to handle SMS to crash when sending an outbound SMS.  While other system apps may be safe to disable, I would recommend against disabling the stock Messaging app.

See this question for more details.