The difference is the quality of the OSS implementation: most OSS ASN.1 tool choke on the enormous 3GPP specs and others used in the telco industry, thus cannot generate 100% valid code.

For some use-cases, you can get by with manually adjust the generated code. That works until the hardware vendors release a new device that use a more modern 3GPP specs and your code start breaking again.

When using a commercial ASN.1 tooling, they often update their compilers to support the latest 3GPP specs even before the hardware vendors, and thus supporting a new device is way simpler.

If I got paid to write an 3GPP implementation one of the things I might do is make one open source ASN.1 stack really good. I've worked on open source projects as part of proprietary work.