It is free and it is genuinely unlimited on volume. Google's own documentation calls Analytics for Firebase "available at no charge" with "unlimited reporting for up to 500 distinct events" and no cap on how many events you send. Skomi is metered and costs money. If price is the whole question, this page ends here and Firebase wins it.
The 500 is on event names, not volume. It is a real ceiling and a strange one to hit. You notice it when a well-meaning logEvent call takes a dynamic string. Skomi's screen names have no such cap, and its props are limited per event rather than globally.
Where the data goes is the actual difference. Firebase "may process and store your data anywhere Google or its agents maintain facilities", and Analytics has no EU residency option. Skomi's app data is stored in the EU, for everybody, without a setting. For a company that has to answer where its users' data lives, that is not a feature comparison, it is the decision.
The device identifier. Firebase stores an app-instance identifier on the device, and Google's own guidance is that app IDs fall under privacy laws and need consent. Skomi's app SDKs also mint an installation id. This is not a row Skomi wins outright, and pretending otherwise would be dishonest, but it identifies an installation and nothing beyond it, is never joined to an advertising profile, and belongs to no advertising business.
History. Firebase caps user-level retention at 2 or 14 months depending on the property. Skomi's window is your plan's, and on the larger plans it is longer than 14 months.
Skomi also measures your website. Firebase Analytics does web through GA4, which is a different product with a different console and a different data model. Skomi's site and app numbers are one account, one login and the same session window.