I've heard from a few members that FR 64s tonearm can be used with High Compliance cartridges, i got the arm, but i've never tried any High Complinace cartridge on that arm, simply because i have different arms for High Compliance cartridges. BUT The FR arm has detachable headshell and optional light counterweight, so it can be adobted a bit for experiments with higher compliance carts. Ikeda-San designed FR-64fx then for cartridges with higher compliance (i have it too).
The Black Widow tonearm does not have detachable headshell or optional counterweight to add mass. This toneam has very thin armtube (easy to damage). The whole combination designed for High Compliance cartridges like Stanton, Grace, ADC ... The effective mass of Black Widow arm is just 3g (this is extremely low mass). Honestly i think it's a very bad idea to use this arm for low compliance cartridges. I'm pretty sure Raul can hear the resonance frequency all the way from Mexico @uberwaltz