You're right, it is quite rare to hear about failed 940 ECUs here. The early LH 2.4 ECUs in the 240s/740s were known to occasionally fail, especially the 1989-1990 pink label ECUs. I expect a number of those ECUs that 940 people mention they've replaced are people throwing parts at a mystery problem without doing a full diagnosis and the ECUs were actually okay.

I wouldn't spend big bucks on a rebuilt one, certainly not as a preventative measure, but if you find a dirt cheap used one then perhaps grab it as a spare. It's always nice to have access to a spare used ECU as well as things like a used AMM, fuel pump relay and a CPS rpm sensor to swap in for testing to quickly help isolate a problem.

You have to realize that a rebuilt ECU is one that may have had significant problems and any repairs or testing they did may not have fully addressed looming issues like a whole bunch of aging capacitors slowly losing their capacity or a heavy load, electrical surge or static discharge that weakened a few components or damaged board traces. A warranty is only as good as long as the company is still in business and you don't mind waiting for a replacement to arrive (if they've even got one) and having to ship back after getting an RMA.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now