They need to have predictable plugins; predictable visitor patterns; predictable use cases. Every hosting company has rules (or very real physical limits), but WP Engine goes a bit further to define what you can and can’t have on your WordPress install in addition to tiered overage pricing to discourage seasonal traffic spikes and local storage usage.
If you are in what I think of as WP Engine’s core markets – they offer a great service with a solid product. Their pricing is competitive in the Managed WordPress Hosting space – and they offer more features than “WordPress hosting” plans from other hosting brands. Their feature-set is unmatched for savvy DIYers, WordPress website developers and/or high-traffic sites that don’t want to worry about hosting issues.
WP Engine also offers a high-end Custom plan that's designed for "large businesses and mission-critical sites," according to the company's webpage. With Custom, you get millions of visitors of month, 400GB of monthly data transfers, and up to 1TB of storage. You must call a WP Engine sales representative for pricing information, though. It's definitely not cheap web hosting, as WP Engine positions itself as a high-end managed WordPress host.