depends on the FX. If both your sound interface and the FX unit has balanced ins and outs and the ground has been wired correctly, just use balanced cables and you’re fine, it should be 100% hum and noise free. If you have some cheap stuff, or just suff that has been wired incorrectly, or you’re using some guitar pedals, then that might be different. As long as cables are short though you should be fine with unbalanced cables. Impedance problems are another thing though.