DolphInN...but we felt being able to directly handle the servers to insert our custom configs and plugins...
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If we were able to 1. access to API, and 2. upload custom plugins, then we'd gladly explore using it.
This API is now a decade old and TF2Center has been running pugs without extra requirements. You can have a TF2Center game on a stock TF2 server as long as it has the map you're trying to play.
The other services don't require custom plugins, but even so, the previous incarnation of RGL pugs had a working serveme.tf API integration and their own custom plugin installed on all na.serveme.tf servers for stuff like team and nickname management.