I agree that there are some subtle considerations...
For me personally it came down to bulk. I don't play piano, so I can get by without the G-70's top-quality 76 keybed. Also I'm not a big guy, and and I just didn't want to wrestle with a 45-lb. arranger.

According to Diki (remember him?) the BK-9 has better mastering tools, and of course the chord sequencer. I have other instruments with the ability to create and store a chord progression, and I seldom use it. It seems like the start of a slippery slope toward playing over recorded music, which so far hasn't been part of my approach to the hobby.

The BKs and EA-7 allow styles with multiple drum tracks. A style like this won't play on the 'G' (although the styles on the G-70, and even older Rolands sounded pretty good, so the improvements are incremental.)

My last area of concern is parts/repair. If you pay a low enough price, I guess that mitigates the risk of failure. But if you buy one and really like it, you had better buy a second one for backup. Because if something goes wrong you may find yourself looking for another board!