Andrew Connell just released an interesting post, which mentioned:
"Customers already dismiss the add-in model... I don't see many people using it today. There's a lot of talk around it, but in reality, I bet Microsoft wouldn't want to admit how many people have built add-ins and deploy them to SharePoint Online in Office 365. Another model would be dismissed even quicker."
App / Add-in model is quite complicated at this stage. The development/troubleshooting productivity is poor. However, the idea is correct: we should keep development outside of SharePoint server.
No doubt, add-in mode on SharePoint 2016 will be much better. Then, more and more developers will start to utilize it. However, I do concern about the latency issue. Response from another (potentially quite far away remote) server, is always much slower than response from another process on the same server. What can we do to resolve this problem? Well infrastructure design and AJAX asynchronous calls help a lot, but no doubt, it could be headache.
So far, I haven't jumped to "App / Add-in model", because, no need. What I did, is to minimize the requirement of full trust solutions.
Four years ago, 100% of the sites I created used full trust solutions; now, 5%
The whole farm is stable, the clients are happy, IT department colleagues are happy, and I am happy!