The thing about that sandbox metaphor is that you can do literally ANYTHING in a "sandbox"...but that carries with it the problem of "what do you create in this totally-open environment?" And without a sense of discipline and a suitable background, the likely things that'll be created will be "bleeps", "blips", "drones" and/or "nothing". So the failings there aren't with the tools...but the user, which is sort of the corollary to another maxim: "Equipment doesn't create success...success is up to the musician alone".
And believe me, it is VERY easy to get lost in modular patchwork to a point where you've lost any creative momentum that might've been present before you busted out the patchcables. This gets right back to self-discipline and technical/musical knowledge, both of which I think are essential to cultivate before stepping into modular for that exact reason. Not saying that you need a music degree and all of that, but if you intend to pour several grand into a modular, you need to know why you think you need one in the first place. And that means that, yes, you know your work well enough to say where it needs to go AND you know the gamut of tools for that work well enough to make ample use of one. Otherwise...well, it's worth knowing what depth of the pool you're jumping into, as you don't want to swan-dive into the wading end, nor do you want to casually hop into the deep end and expect that bottom to be right there...when it's NOT. Most people know what to do in that situation...but when confronted with the "modular or not?" choice, the default seems to be to start chucking attractive (and expensive) modules into a box. And to quote Rocky J. Squirrel, "That trick never works!"