Complex sentence: All the sites in a site collection will be stored in the same database, which can become too large to effectively back-up or DR, and bespoke development using the same Web Application and Application pool can bring a company wide internet down.
Split into simpler sentences: All the sites in a site collection will be stored in the same database, which can become too large to effectively back-up. Moreover, bespoke development using the same Web Application and Application pool can bring a company-wide internet down.