There are several potential difficulties that shared hosting customers can face if they are trying to use for a project to which it is ill-suited.
The biggest problem is the lack of computing resources — memory, storage, bandwidth, processing cycles. If your site traffic gets too high, or if there is a large volume of concurrent requests, page load times can slow to a crawl. This negatively impacts both user experience and SEO ranking. If that starts to negatively impact other shared hosting customers on the same server, your hosting company may throttle your access — making the site unavailable when it is drawing the most attention.
While it’s true that you can often upgrade directly to a VPS plan at that point, it’s also true that the lag between the need to upgrade and actually upgrading can cost you in ways which are hard to recover from. You don’t want to turn people away with Server Errors just as you are starting to see success.
If you expect that kind of traffic, or you need that kind of traffic for your site to be viable, then you should just opt for a VPS plan in the first place.
