FYI: soon after this post, the exams were back in RHLS for enrollment, and I had the opportunity to take both of them 
EX425 took me more time than I expected (so fortunately, it was 4 hours instead of the 3 hours the Red Hat website mentioned), but after all you should be able to ace it if you took the DO425 training, know your way around OpenShift or have equivalent experience.
Try not to learn all the different YAML configuration files by heart (unless you have an absolute photographic memory), but instead, remember how to export existing objects to YAML to edit them, or know which parts of the official documentation contains the examples you need to build the config you need.
Also, pay close attention to the position you insert your modifications: one task took me way too much time checking why the behaviour was not modified, until I realized I put the syntactically correct configuration at the wrong position in the file
Good luck!
