I came across what seems to be another vocabulary only ‘predictor. The ‘CEFR checker’ is most probably drawn from the English Vocabulary Profile. As with the ‘Write & Improve’ website’s CEFR level predictor, I put through 10 sentences that contain C2 grammar points. Both of these gave me a prediction of B2. The same sentences put through the English Grammar Profiler app here at EnglishGrammar.Pro gave a prediction of C1-C2.
Of course, this is a biased and small sample, but it says something for adding an extra criterion to CEFR level predicters.