Most programming languages themselves are OS.
Most scalable software is nowadays hosted/running on some unix-based operating system.
Many of them are OpenSource.
Nearly all the time software products have a foundation of OS software.
Fundamental functionality is most of the time handled by OS libraries. (p.e. SSL-handling by OpenSSL)
You can be able to scale your product fast and cost-effective as there are no licensing costs.
Note: Larger OS projects sometimes offer a paid-for tier that provides additional enterprise-grade functionality and support.
- CS solutions can have more capabilities - might be less admin-work and easier to integrate and maintain
- You can get locked-in to one CS-solution.
Companies like to build their own IT ecosystem that will be integrated best with their own toolset.
Interfaces to 3th-party tools might be neglected for more profit. Just look at Apple of Microsoft..
- When evaluating CS software, you should mind that some companies might promise more than the product can actually deliver.
Sadly - not everyone values moral decisions over higher sells.
I've had to deal with such whitewashing multiple times in the last years.
- Custom addons/plugins are supported by many OS products as extensibility and ability to customize functionality is a fundamental mindset in the community.
- The behavior and documentation of OS software can be more transparent.
- When using OS software - the organization depends on its internal specialists.
- When using CS software - the organization depends on external specialists.
- Larger/matured OS projects will have a larger community.
This can be useful to find external personal that has experience with it.
- You should check if an OS project has multiple contributors! See: GitHub