Differences between US and Singapore that actually require work before arrival and are worse for the US:

Belarus, Cote d'Ivoire, Cuba, Djibouti, Ghana, Guinea, Iran, Kenya, Myanmar, Uganda, Vietnam.

Everything else is either visa on arrival or the same between the two (or worse for Singapore).

I'd like to see Cuba and Iran. Maybe Vietnam. Kenya, I've been to, although at the time the visa was granted at entry (though it was not free). And not all of the e-visas are free.

But in the grand scheme of things, my US passport makes it harder in four countries I would ever have an interest in going to unless there was something spectacular I don't know about. Seems like it's a pretty minor thing, not like it was in the early 90s when I needed a Taiwanese visa that required going to a consulate.

Pretty much all "rich country" citizen issues boil down to a handful of highly restrictive states and a few that have particular issues with the US government. It's not really limiting me the way that, say, an Indian passport would.

USA not qualifying because the USA is very hated by locally powerful countries that don't like that the USA doesn't let them run amok, e.g. Russia, China, Iran.