Production-grade web app without advanced build tools? Depends.

CSS classes not scoped and starting to leak? You hire more frontend developers and because there is no type system we get critical exceptions? And no automated testing to discover them?

Correctly handling hyphenation of user-generated content? Safari decided to handle audio differently in the latest version and you have no polyfills? iPhone decided to kill the tab because of memory pressure, because someone uploaded an image with exotic properties, and you have no cdn service like fastly image optimiser to handle that? Support for right to left languages such as Arabic? The backend returned a super cryptic response that actually originates from the users private firewall?

a11y requires you to use resizable browser text, and someone is using google translate chrome extension at the same time, and you can’t possibly know how the layout of the page will look like?

Some Samsung devices bypass browser detection completely and you don’t know if the user is on mobile or not? localStorage.setItem will throw an error when the device is low on memory, etc etc…

Once you get to a certain scale of users, even the simplest of tasks become littered with corner cases and odd situations. For smaller scale applications, it is not necessary to have a very wide tool arsenal. But if you are facing a large user-base, you need quite some heavy caliber tools to keep things in check.