Ah! And I was racking my braincell trying to think of any fundemental difference between 10 and 11.buddard wrote: ↑02 Dec 2021
That is very interesting! I’m assuming there’s no specific error message?
Did you omit the Placeholder widget on the back panel by any chance?
It’s no longer required according to the specs and Reason 12, but Reason 10 (and probably Reason 11) will still fail to load an RE without it…
Yes, it doesn't have the placeholder widget, now you mention it. Good catch!

(I'm not putting it back

So given that SDK4.3 (and pretty sure 4.2?) doesn't require the Placeholder to be present, on those grounds alone, 4.3 is therefore not, by default, compatible with Reason 10, as Reason 10, it would seem from your theory, still looks for it. The device quietly fails to load (no error message, literally just doesn't appear in the rack) if it's not present. A new device under 4.3 would have to include the Placeholder purely to be compatible with R10, thus it becomes a specific developer choice to either include it in a new device to ensure the most backwards-compatibility, or merely forget to remove it from a pre-existing RE starting template and get that extra compatibility coincendentally.
Anyway, from a design perspective, especially with 1U devices, where the placeholder was always a MFPITFA, I would have thought most devs would be glad to see the back of the damn thing...

FWIW, again assuming your theory is correct—and I'm inclined to think it is—Reason 11 RE's do work without the placeholder: PH typically don't support more than 2 major revisions of Reason per SDK update. SDK4 is R10+ only, for example, and so here it is on R12, a third gen SDK4.x revision. So I'm guessing there might be a level of intent here to drive forward upgrade/+ progress: that default support for new REs is still two Reason versions, 11 and 12, but there is a workaround to support R10, if you know how. A theoretical SDK5 will almost certainly remove support for both R10 and 11.
Custom display performance sucks on R10/R11 anyway.
If you're using heavy draws on custom displays, recommending R10/11 is hard. R12 has access to so much more processing power/resource sharing to do the draws. I don't think FPS is higher (still ~20fps, maybe someone has the tools to check), but anyone can compare the Harpe Laser II performance with CV input fast-driving the RGBLS stuff on R11 and 12 during heavy song playback and see the difference for themselves.