TES modder here. 
The quest and dialogue system on the Marketplace is nice but can be limiting sometimes in its current state, so I had to modify it heavily to get it up to scratch in a complex narrative RPG. Buying assets saves a lot of work on a certain thing but you do end up doing a lot of legwork to make it unique.
For example, in my quest mods, I can use a GetStage function on scripts to check the quest stage. In the system, it exists within dialogue but it doesn’t exist for use in external Blueprints so I added that manually. Templates are templates, not full systems. It’s up to you to heavily modify and add to them.
I use assets 90-95% of the time, having custom made assets where necessary. If I want a generic set of mountains and someone already made these exact mountains, why reinvent the wheel if it means paying more for exactly the same thing? On the flip side, if I want a UK style school modular set, I’m not going to “settle” for a US style school modular set.