In terms of difficulty it is similar to 809, only that it has the concurency, jdbs questions replaced with modularity.
Questions about java modules where mostly ok, some of them included questions about command line parameters for different tools: javac, java, jar, jdeps, jlink
For the exam, I read almost entirely The Java Module System written by Nicolai Parlog, Mannning Publishing. I bought it directly on their site, it's early access program now. This book has a very good difficulty curve, it takes from the basics and goes into deep details.
Trust me, you will not understand modules only from reading online tutorials, they rarely explain the important details like module graph resolution, reading. Without them you are stuck not understanding why JVM throws module not found error even when you have added it in the module-path.
The exam had tons of questions about Lambda, Streams, Functional Interfaces and Comparator interfaces. This can be safely considered the core of the exam. If you have taken 809 exam, take this one as well. You are already 50-60% qualified for it.
And most important, read everything Mikalai Zaikin has written about 1Z0-817 exam
Congratulations, Viorel! 88% is actually an excellent score given that there are no resources to prepare for this exam. Great job.
80 questions for an upgrade exam is huge. May be Oracle is testing new questions.