Spot on. If your audience is basically anyone outside of the HN echo chamber, then default bootstrap is
just fine.
I used to build big internal webapps for big companies. My design skills are only slightly better than those of a drunken orangutan.
After discovering Bootstrap, I used it to build a static prototype of a new system I was pitching to one of my clients. Not only did they spend nearly 10 minutes gushing about how clean and gorgeous and minimal the design was, they approved the project immediately and suggested that I re-skin an older webapp with the same theme.
After I told them that I'd used Bootstrap to build the mockup, they said "Oh yeah, I think one of our other teams is using that too. Good stuff!" I still got credit for being a design genius.
If you are building an app for attorneys, pool cleaners, CPAs, gardeners, insurance agents, property managers, window washers, dog walkers, code enforcers, self-storage facilities, mailbox companies, dentists, or payroll supervisors, then in 99% of the cases, they have never even seen a UI half as beautiful or usable as as the bare default Bootstrap styles.
Niche software is, as a general rule, terrible. It's clunky to use and looks even worse.
So, again, if you're building software for designers or people who hang out on HN all day and get annoyed by UI uniformity, then by all means invest a lot of time and money coming up with something unique. But if you're selling to people somewhere else out there in the greater world, that time and money will be 1000x better spent in getting to know your customers, finding out how to reach them most effectively, and creating a compelling offer.