I'm not saying it's a bad technology choice for a new company when all you need is to build a CRUD web app fast, but I wouldn't use it for anything that requires you to serve >1MB of JSON API responses or anything that requires concurrency, substantial IO, lots of API interactions with external services, low latencies, lots of memory or compute heavy computations, etc.
We use grape-api for some of our API endpoints. I needed to set some custom cache headers for some GET API responses and was looking into gems that do that for grape. They have a list of 6 libraries in their docs [1] that they recommend.
Their newest commits were 4, 9, 9, 10, 8 and 9 years ago, respectively. This is just one example, but I would bet half of the libraries in our transitive dependency tree haven't seen any maintenance in this decade.