In theory ice is not detected well by weather radar so it might be argued that a shower of pure hail should not paint well. However as hail only occurs in conjunction with thunderstorms and in practice there is always sufficient (perhaps supercooled) water present in a thunderstorm to show the strongest returns you will ever see. If you're low level with your tilt low so as not to see the CB itself you'd see nothing but ground clutter so in reality the CB above will always give the game away. At low level the hazards are microbusrsts/windshear affecting airspeed and hail damage. Heavy rainstorms can be associated with microbursts too. At med level hazards are heavy turbulence/windshear (much less dangerous at altitude) and hail damage
However at high level where the CB contains nothing but ice (no supercooled water) and hail they can be difficult/impossible for radar to spot although imperative to avoid due turbulence (large size hail unlikely at high altitudes). This is a particular hazard at night or when CBs are embedded in cloud/haze and cannot be seen by eye.
There is no particular reason to avoid rain showers per se except that if heavy they are often associated with turbulence/windshear.