I prefer flatter fretboards generally, and love the 12" radius boards on three of my G&L basses. For a few years, I had low-level GAS for a JB-5, and if I'd ordered one, would have specified the 12" radius. But without a band, I couldn't justify the expense.
Then I saw a used JB-5 available locally which had the stock 9.5" radius. I was able to test it, and thought I was unlikely to buy it because of the radius. But it felt great, the price was right, and it had a nice lightweight empress body in Clear Orange, which honors Gritty, so I went for it.
I think it works for me because of the slim rear profile (which you noted), Plek-leveled frets, and the nicely rolled fretboard edges. It's very comfortable overall, and I enjoyed playing it last night on "stage" (actually a garage floor we shared with an old Chevelle SS on blocks, but hey, it's rock n' roll). Would I prefer the 12" radius? I think so, but so far I've been happy with the neck.
By contrast, the Fender Deluxe Active Jazz V doesn't have nicely rolled edges and I think it has a thicker rear profile, so despite the 12" radius I'm not crazy about that neck. I prefer my JB-5's neck, no question. (I do also prefer the G&L's narrower spacing.)
G&L has made a big move to 9.5" radius fretboards as standard on most guitars and basses, with the L-2500, M-2500, and MJ-5 as some of the few exceptions. I think they've done this partly to split the difference between their former 12" and 7.5" options, and also to make their instruments feel more familiar to those used to Fenders, most of which have 9.5" radii. In fact, I came to G&L after a few years of playing mostly 9.5" radius Fenders, and at first the 12" radius boards didn't feel as natural, especially at first grab. But I soon realized I played a little better with the flatter boards, and started to develop the preference.