United doesn't offer an international first class product anymore. United First is United's North-American* premium cabin product. The other UA premium cabin products are United Business and United Polaris​ business class. Each product serves a dedicated market.

Since there are currently no markets that offer more than one UA premium product, it's technically impossible to rank the different products vs. each other. Sure, United First has fewer amenities than Polaris business class, but they're also serving vastly different markets. Other airlines also differentiate their premium cabin products based on the market served (e.g., LH).

* with few exceptions such as EWR/BOS-SFO/LAX