One of its strengths is represented by the quality of the source code, developed according to best practices and the BDD approach (all its features are covered by Behat scenarios).
Sylius benefits from the wide availability of Symfony bundles, which allows it to be expanded without compromising its operation.
Setup time for a working project are minimal, and familiarity with the Symfony framework drastically reduces development and customization times: although more mature platforms may offer more out-of-the-box functionalities, the buzz around the project in the developers' community portends a rapid growth in the features offered.
Moreover, the core team offers a Plus version, which consists of some specific functionalities and the support of a Customer Success Manager. It seems to be a valid alternative for small businesses who want to migrate from Magento 1 to a flexible platform.