The standard ADF's RBI display also requires an additional layer of integration on your part ie you have to convert a relative bearing + heading into the equivalent VOR display: displacement from a desired track.
Even an RMI takes a little more concentration than a CDI or (better) HSI.
Every unit of your brain 'ability' that is used for something other than flying & controlling the a/c will detract from your 'normal' ability to do so.