It's not about debugging tools.

Different, high level software designs (i.e. architectural designs) which are normally imposed by the game engine, have different probabilities of the developers who are making the code for those to produce bugs, because of lots of factors including things like of how they approach error validation and handling in the engine itself and in which domains does the engine leave the most freedom to coders and which ones does it leave less - some things are pretty safe to leave in the hands of even bad developers, others are not.

The example of multi-threading in Unity should've been clear: put a game engine that doesn't impose a single thread pattern in front of somebody with little or no experience in multi-threaded programming and you will have a huge rate of bugs (mainly critical race conditions) and as it so happens most developers out there have little or no experience in multi-threaded programming. Yet multi-threading can yield far more performance in modern CPU since they're all multi-core. For that specific game engine a software architectural choice was made to go with a structure that is not as performance but significantly less likely to lead to a higher bug rate when used by the average coder, probably because Unity targets less experienced coders.

Good Senior Designers and Technical Architects don't design the high level structure of the software for themselves as coders, they do it for the kind of coders that are likely to be coding for it.

Of course the developers themselves also have different capabilities and hence different baseline rates of creating bugs, hence why I said "both".