What is the threshold to prefer polyphase channelizer over FIR filtering?
A polyphase channelizer is not a special kind of filter. It is a structure that works well when using filters in multi rate settings. Polyphase is a sampling rate conversion method that leads to efficient implementations that are useful for building filter banks. The efficiency comes from only having to design one filter. The downside is that the extracted channels have to be uniformly allocated and the signal sample rate has to be an integer multiple of the channel's bandwidth.
Use a channelizer for 2 more channels that are uniformly allocated.