One of the amazing facts I realized after learning Houdini was how narrow my vision was, regarding what it was possible to do in 3D. Adjectives like 'painful' - 'not painful' adhere too.
My vision here is affected by my own professional experience, as I worked mostly with 3dsMax, Maya and Luxology's Modo.
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Either 3dsMax and/or Maya were the preference by all the studios I worked for in the gap 2012-2019 and I couldn't believe how could some tasks be so painful in 21st century. I asked around, but most of my colleagues answered something like 'that's the way it is, better get used to it'. Years passed and I realized they just preferred to remain within their area of comfort, or they just missed the chance to look for some other 3D package. As a result, they kind of accepted the stablished pipelines without further questioning.
If you got a different answer back in that period, I'm happy for you, as it means you explored more 3D Digital Content Creation (DCC) alternatives earlier.
Anyways, I started my own research within that period and learnt Modo in my spare time. This confimed my suspicion: my vision regarding what was availble for 3D content creation was limited. Bumping into Luxology's Modo was awesome, as I could feel I had met a modern (DCC) package.

Today, alternatives like Blender are pushing hard and conquered indie and bigger studios, which is great (I never became a Blenderian myself, though).
If you are wondering, 'well let's assume you are right, and it is clear those stablished 3D packages didn't suffice. Does it mean studio managers and Technical Directors were wrong by choosing them?'
The answer is 'yes' and 'no', but that's another topic we may explore in a different post.
The key factor that makes Houdini hard to learn is the unique approach it uses to get things done: procedurality. Let me elaborate.
Most 3D packages are content creation-oriented, hence, the creation process is designed as a one way road: 'You know what needs to be created beforehand, so you pile up actions till you get it done, good luck with asking for changes in late stages'. By nature, this is destructive approach, in the sense that any change involves redoing work.
On the contrary, Houdini is process-creation oriented. That is the most simplistic way of explaining procedurality. With Houdini, you won't create a House. You'll create a process to generating Houses instead.