Playmaker is not designed to replace all text-based programming in Unity but replace it in the use cases where it excels. For what most people want to do most of the time in this context, a visual language is easier to write in, debug in and maintain.
I agree with you that Playmaker isn't designed to replace all text-based programming and I believe that the tool works well for what it was designed for, namely to allow artists to do a lot of stuff without having to wait for the programmers and to allow people to do all of that, for lack of a better word, "brainless" programming that most programming actually is.
But when it comes to debugging and maintenance, I'm not so sure. Especially when it comes to debugging; I see no reason why that should be easier with a visual language.
It's easier to debug because this kind of logic is asynchronous over a human-readable lengths of time. With a visual language you get at-a-glance information about what step is currently playing and it's much easier to see what's going wrong and why.
It's also not about "brainless" programming or for artists. I find that quite an elitist attitude. It's about getting to the core of the problem without the unnecessary boilerplate or engineering requirements that complicate solving the immediate problem in the quickest, safest cleanest way.
At some point of complexity you end up fighting the limits of the system. When you hit that point then visual programming really slows you down. Being able to use text-based programming as a fallback is a real advantage there.
It's the same debate that people have with DSLs. Your DSL is turing complete. So's C++. C++ is more flexible, faster and more powerful. It has powerful, mature compilers and debugging tools. So clearly you should always use C++ instead of any DSL.
