My biggest concern with the approach of picking something you have an existing audience with or is in some corner of your wheelhouse is that it leaves a lot of opportunity outside.
Is there not a huge competitive advantage in selling to underserved markets? You may believe market saturation is "bullshit", but it's much harder to get noticed in an already crowded field.
There seems to be two fields of thought on researching and marketing a bootstrapped product:
1) SEO and niche marketing. This is the sort of thing that Rob Walling talks about. Basically the idea is you research through Google keywords that you can rank highly for and build a product around that, regardless of what you yourself are good at. The problem with this is that the internet is full of scumbags and many online audience "niches" are already taken. PPC is extremely competitive as well.
2) Content marketing and "become famous". This is the approach that I think Amy and Ryan Carson recommend. This has a longer path to success, years in some cases, and is highly subject to survivorship bias. To be frank, it's hard to get noticed in our little world unless you're a) an extreme extrovert and b) extremely charismatic and/or c) really lucky.
There's a middle ground that Patrick McKenzie occupies that combines traits of #1 and #2, but also introduces the free radical of underserved markets. Actually, the reason for his success is that he initially started with an underserved market, and then used that to gain SEO knowledge and internet fame to leverage #2 to some extent.
I've emailed with Amy on this regard, her tone has been basically "Well, it works because he's Patrick". I disagree with this sentiment because it sets up a "Golden Calf" mentality that we mere mortals can't follow the same path.