Would be nice if we had an OnSubmit/OnSaving event on the form controls that we could use to do validations. Kind of like the PreSave function on sharepoint forms. If the event retured true powerapps would wubmit the form , otherwise it would not. We could put validation logic in the event and show/hide custom error messages as needed,
As it is now, we need to scatter this logic throughout different controls. With various nested dependencies it can be difficult to understand where such logic should be declared. Providing a single place where such logic could be declared would make it much easier to develop and also much easier for a new person to understand what the form is doing.
I see the Validate method can be used to peroform some validations based on the schema, but there are other validations we often need-- which cannot be expressed in the schema-- that would be ideal candidates for a OnSubmit event. Particularly when there are a lot of interdependancies between fiels on a form.