hmmm interesting. In a gasoline engine with high temperatures, NOx is produced, but it is fixed by the catalytic converter. So I guess that's not possible with a diesel, so they have to lower temperatures. That must lower the efficiency of the engine because (I think) the temperature difference governs efficiency.
Currently only smaller engines are EGR+DPF only. It takes having an SCR(&DEF) system to reduce emissions to legal limits on most larger engines now and the technology has been combined to have an engine with EGR+DPF&SCR(&DEF).
There are issues with engines and the active regeneration that has to occur in order to clean the DPF(basically engine has to get very hot and burn out the buildup) and also issues with DEF, which is an ammonia solution, both with the electronic dosing units failing and solution purity.