Your content can't be archived properly by the Internet Archive.
Your content can't be accessed by anything user-agent less sophisticated than a full-blown browser.
Your content can't be indexed by a search engine that doesn't have the resources to crawl using full headless browsers.
Your content won't work with tools that attempt to scrape and expand content (think URL expansion on Slack, Twitter, Facebook etc).
(There's a very high chance that your content sucks in widely used screen readers as well)
HTML for content, CSS for presentation, JavaScript for behavior enhancement. Not everything has to be built like Trello.