Hi Vincenzo,
I would say that there is no one correct answer. It all depends on application architecture and purpose. I think Fiori type apps with 1 use case and only a few screens are better on 1-tier architecture. That's what I think is also SAP's recommended practise.1-tier makes application architecture simpler, so that there's fewer layers that can break.
Once you start integrating multiple systems, especially non-SAP, 3-tier architecture becomes more valid. I wouldn't add another layer just for validations or error handlings. These would have to be done on the backend system in any case. 3-tier architecture is also useful in case you have to use existing services and cannot modify them when need be. 3-tier architecture also helps in keeping the business logic out of the client, which makes it easier to reuse or replace parts of the services or application.
Of course, you also have the option to combine both approaches...
Regards,
Kimmo