Any router of course will have some flex. I have an older CNCRouterparts Standard machine which is the same basic construction as the PRO machines with the 8020 Aluminume extrusion gantry.
Here is a sample pocketing 6061 aluminum with a 3/8 single flute HSS cutter at 16,000RPM .050 stepover, and .250 DOC.
While I have never measured the flex on my gantry, I can tell you that the pocket in the below video was smooth to the touch.
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And this picture shows the finished pocket.