I had been working with ruby for the last 6 years, although my first experience with it backs to 2016. I used it in distinct stages of startups, in Canada, San Francisco and Sao Paulo. (from pre-seed to IPO)
From my experience there's a lot of room to build robust applications using the vast and mature ecosystem of ruby. I had the opportunity to work with api only, with react on rails, pure erb with vanilla js. Each with some pros and cons.
One of them was really interesting, based on lessons from Domain-Driven Rails, which I much appreciate for scaling busineses with high complexity in terms of rules.
Tooling today is quite mature too, even from simpler ones as VSCode or even RubyMine (unfortunately paid as opposed to other editions... Subject for another thread tho).
I'd highly recommend Ruby as a starting point for founders, cause there's no silver bullet indeed, however the speed and flexibility of a full featured Rails app today is impressive. Both for coding core app features, scripting and even fastly mungling data (let's be honest, it's quite common pivoting in early stages) using the rails console.
Yeah, single thread and stuff out of the box, but I'd guess that a startup having such a requirement in day 1 is 1 in a hundreds of thousands cases (I can be wrong, want your thoughts too folks).