The British Council recently funded a study comparing the IELTS Academic Test and the Duolingo English Test. You can read the study here. The authors of the report suggest that the DET has some weaknesses. They conclude:
Our analysis demonstrates that, compared to IELTS, DET test tasks under-represent the construct of academic language proficiency as it is commonly understood, i.e., the ability to speak, listen, read, and write in academic contexts. Most of the DET test tasks are heavily weighted towards vocabulary knowledge and syntactic parsing rather than comprehension or production of extended discourse, though the recent addition of Interactive Reading addresses this lack somewhat.
But they do note that:
Scores on the two tests are correlated, which might suggest that DET is a reasonable substitute for IELTS, given its accessibility and low cost. Of course, knowledge of lexis and grammar are essential enabling skills for higher-order cognitive skills, and a test that focuses on these lower-level skills can be useful for making broad distinctions between low, intermediate, and high proficiency learners. However, potential test users should be aware of the limitations of DET in terms of predicting academic success.
The study was done by researchers working out of Georgia State University.