Shopware is mainly based on Symfony, not Slim. Maybe you should also take a look at Shopware 6 which is definitely a big milestone - With Shopware 6 you’ll use Twig, Sass etc. Pitfall: Shopware 6 isn’t released yet for the public, but will be this month. However you can check out the dev preview: https://docs.shopware.com/en/shopware-platform-dev-en
In my opinion: Shopware and especially Shopware 6 is one of the best open source systems on the market - Great source code, Symfony, Vue.js, Sass etc. are just a few features i really like in Shopware 6.
For Shopware 5 in my opinion the biggest pitfall is ExtJs which is used for the backend and a few other things. So you’re a bit limited in case of developing backend plugins / user interfaces for the plugins. And i think ( i might be wrong ) Shopware 5 is still mixed with the Zend Framework. But i am not 100% sure, they maybe switched 100% to Symfony. But - Still one of the best e-commerce open source systems in my opinion - I worked with several systems like Magento, Woocommerce, Shopify, Opencart, Prestashop etc. But Shopware 6 is by far the best 