Yes, now I can see your point. Haven't considered this in depth before. Maybe because we have so low expectations in software, when something mostly works without visible manifestations of failure we tend to consider it better quality than actually it was.
But, yes, the absence of failures should not be taken as proof of absence of bugs.
It's essentially a hardwired analog computer (note the lack of adjustment points other than 'idle RPM'), as long as the external contacts are clean, the components are good and within tolerance the unit should work. There is nothing to adjust.
Modern ECU's have many more sensors to deal with, so higher data rates and many, many more fault conditions that have to do with emissions and preventing engine damage, these old D-Jetronic units were very impressive for their time, plenty of them are still running today which given the parts count is a small miracle.
In a way they are so reliable because of what they can't do, as long as the input parameters are roughly where they should be and the crank position input is accurate the engine will run. But whether it will run efficiently or not is a different matter and accurately setting these up without the right gear and manuals is next to impossible. So most parties that repair these will do a 'so-so' job, good enough to get the car back on the road but far from optimal. But the old battle-axes that you'll find these units in never were models of fuel efficiency. In contrast to this a modern ECU has exactly zero analog processing beyond digitization, it's all software. And where at all possible that software is not engine (beyond type and pre-set config) or drivetrain specific (though they do tend to talk to the transmission to get more information about the driving conditions) because that would require a lot of extra work per engine.
The most that you'll find is that the ECU has some knowledge of the serial numbers of key components (for instance the drivetrain) to give some anti-theft protection. Other than that it's all auto-configuration until some parameter goes out of spec and then that expensive check engine light comes on.
But, yes, the absence of failures should not be taken as proof of absence of bugs.