As someone who's done his fair share of ExpressionEngine development, the template system is a source of endless frustration as soon as you need to do anything

As someone who's done his fair share of ExpressionEngine development, the template system is a source of endless frustration as soon as you need to do anything remotely complex on the page or deal with user input. It is a great choice for blogging sites where a group of editors publish content to a site in predefined channels, but really falls down if you push it too hard. Template processing is slow, processing order is wonky, and it's very easy to create N+1 query loops that can only be avoided by writing custom SQL queries/php. They also made some poor database structure choices in certain areas. Mostly though it just boggles the mind why they put so much effort into creating a regex-based templating system when the app is already just PHP. Still, probably the best CMS I've used and it certainly has its uses. I just wouldn't use it to develop anything complicated ever again.