Yeah, fundamentally that's the main issue. This thread wouldn't exist if C4D had "Hollywood" quality pyro, liquid, particles, smoke, terrain, cloud, etc. etc. built right in. Where Houdini is dominating is in having those effects available in-the-box, and for most home-based users at a $200/year price point. In addition, from my early tests, Mantra yields better looking results than the Physical renderer (although at this point it seems like the majority of users would prefer Redshift or Octane anyway).
Which brings us back to the learning curve which is (at the moment) Houdini's weak spot. If SideFX is smart, they'll do everything in their power to simplify the workflow for newcomers while retaining the complexity and depth that veteran users want.
For what it's worth -- I've been messing with Houdini apprentice now for a little over a week and I feel like I'm getting my head around it. The basics (importing models into Houdini, texturing, animation, transform, deformation, basic physics, etc.) are not terribly difficult to learn. I think where things get nutty is when coding VEX expressions gets in the picture. Reminds me a bit of AE Action Scripting; everyone knows Wiggle (x,x) in AE, but very few get into heavy duty scripting beyond that.
If Houdini turns out to be functional and capable of yielding good results without the need to become a coder, I think I'm sold. Having said that, it makes 100% sense to do some stuff in Houdini and some stuff in C4D (or Maya, or Blender, etc.). I've been involved in audio/music production for a number of years and I routinely use both Logic Pro and Pro Tools side-by-side, so it makes sense to apply the same reasoning to these apps as well.