I've done a few experiments with manipulating Graal graphs myself.
I do get a sense of slight confusion in this talk over whether Graal is easy to understand or not. It's clearly easier than C2 but this is not a high bar to reach and is not really related to what language it's written in. I'd say Graal is still harder to understand than LLVM, despite LLVM being written in C++, mostly because the LLVM guys really do believe in documentation and building an open source community. The documentation for Graal is sadly near non-existent. There are academic papers, usually undated of course as is normal for such papers, so it can be hard to know how fresh the information is. There are talks and blog posts, which are helpful but are often kind of repetitive if you've seen earlier talks.
What there isn't is anything like this:
LLVM has detailed tutorials, design overviews, descriptions of each pass and so on. This is one reason why LLVM tends to attract a lot of compiler research and has such a thriving community. Graal is good, but it's mostly done by Oracle and researchers from JKU Linz. I wouldn't say it has much of a community (check out the traffic levels on graal-dev for example). It doesn't even have a website or even a public issue tracker, which is a downgrade in understandability from existing Hotspot compilers!
Graal could give LLVM serious competition as a testbed for compiler research, but in its current state trying to work with it is just painfully slow. Even very basic questions like "how do I swap out this node in the graph" or "how do I insert a node in between two others" or "how do I enable assertions that will check if I mess up the graph" require you to learn by reading the existing code, which is hardly commented (most classes have no javadocs).
I used to think that Graal was like this because it came out of university research, so the developers were judged by metrics like how many papers they published. But LLVM has the same origins, so I guess it's really just a matter of project culture. That may prove hard to change.