Let's say someone asked me to build them a youtube clone with a lot of customizations to support a whole bunch of wacky things they want to do.
How do I decide if it's better for me to build from scratch vs. build off a white label or open source product?
Because of my past work experience, I am biased to build from scratch because then I will know the intimate details of what I'm building. This lends to better post-release support and scalability. I never feel comfortable working on off-the-shelf solution for which no one on my team has prior experience working with. I'm not quite sure what I'm getting. I have no idea if it will do all I want to do, how well documented it is, and whether I'll shoot myself in the head 6 months down the road.
There are so many off the shelf solutions I've never used, how do I evaluate them all before I decide whether it's a good idea to build from scratch?
What if I'm building this for someone on a tight budget, but they also expect a lot of support from me down the road? (ie. buying a solution fits the budget, but i'm worried about supporting it afterwards)
In building a clone i would be more comfortable going from scratch because you have a great and proven example to work towards. (The ultimate spec!) If you try to eval/implement some other system to match an existing product you'll be much more likely to run into serious headaches when the base code doesn't work towards your end goal. i also agree with all of your maintenance considerations.
Also, evaluating systems is time consuming and not foolproof. i once spent a month deciding between a 3rd party grid and in the end it was basically a roll of the dice. i evaluated a dozen products for a month. How how much functionality could i possibly touch? 5% maybe.