Move a site hosted on IIS/MS SQL Server across to LAMP stack I have a Drupal 7 site which is currently hosted in a Windows servers environment (e.g. IIS and MS SQL Server). The site needs to be moved over to Platform.sh, which is using the LAMP stack.

I am wondering if there are likely to be any issues with doing this? For example, is it possible to simply export the existing database from MS SQL Server and then import it into MySQL on the new site using drush?
There shouldn't be any issues as such if you properly follow the steps of Migrating a Drupal Site. You can find detailed steps on How to Migrate a Site.

This documentation provides a detailed documentation on Backing up and migrating a site.

Regarding taking a database backup of Site, you can checkout Backup and Migrate Module for quick and easy database backup of site. Here's how you can quickly setup Backup and Migrate module and backup your site databse.