Heme boards were a bit tougher than I expected - just a tad more difficult than the ASH ITEs. Maybe it's my background, but the exam felt overrepresented in benign heme questions. Gotta know coagulation extremely well, rare bleeding disorders and associated conditions, von Willebrand, Hgb electrophoresis. Must've had 10 questions on MAHA. Need to absolutely commit lymphoma immunophenotyping to memory. Plus an unusual amount of Punnett squares, but not overrepresented in the amount of hereditary cytopenic conditions like the ASH ITEs usually are.
I thought there was relatively fewer questions about leukemia (which were all very basic anyway) and myeloma, probably because the fields are changing so rapidly. The transplant questions seemed more conceptual than clinical; likely unfair for non-transplanters. Maybe 1-2 questions only on CAR T and no questions on lymphoma/myeloma bispecifics (actually, very few questions on advanced therapies for relapsed disease generally) but this may change. There was an expected amount of unfair molecular/pathophysiology questions, which you knew or you didn't.
There were both overlong question stems (sometimes the answer doesn't require you to look at the question stem at all, and sometimes the correct answer depends on noticing a small detail) and some that were extremely succinct - in short, challenging and unbalanced.
I'm not sure any particular resource would've prepared me fully. The ASH review series is the best foundation. I only supplemented with ASH SAP questions, and by doing a lot of clinical heme. I tried HOQbank which I thought was unnecessarily detailed, focused on the wrong things, and was outright incorrect/outdated sometimes; maybe it's better for onc but I doubt it. I also did a little bit of BoardsVitals, which I quite liked, but didn't stick with it.
My advice would be to take copious notes on the ASH review series, and do a qbank - otherwise, learn consistently and thoroughly throughout fellowship. The exam may be hard if you don't have a general heme clinic or two, or you don't participate in tumor boards/conferences to learn about more esoteric presentations/newer literature. If so, you'll need to have your ASH ITE score guide how much additional studying you need. Otherwise, I wouldn't sweat it too much - just remember that many questions are experimental and ultimately thrown out.