MySQL moved away from MyISAM in 2010, but plenty of WordPress, WooCommerce and older store databases are still quietly running on it. Table level locking costs you exactly when you are busiest.
Hooks are why you can customise CubeCart and still upgrade. The hook editor, extension ownership in the registry, and a Smarty security policy that changed four times.
WP-Cron is not a cron job. It runs on visitor page requests, so a page cache and a CDN can silently stop your scheduled posts, backups and WooCommerce renewals. How to fix it properly, in the right order.
If some visitors see your new server and others still land on the old one, this is why. Lowering TTL fixes it, but only if you do it a day in advance.