In the season Boro went up, they conceded 31 goals all season and kept a record 23 clean sheets. In the prem, we were 9th in the table for goals conceded but got relegated. It's all well and good having defensive structure but it's a completely different story trying to do the business at the other end as well.
in the 27 years of the premier league, only 3 of them didn't have a newly promoted side in the relegation places.
I think it's under estimated how much of a gulf in quality there is between the championship and the prem. What are considered match winners in the championship are bang average players in the prem. You only have to look at players like Dwight Gayle, Jordan Rhodes and Tammy Abraham to establish how much of a step up that really is.