With Assetto and Fanaleds (as of this post 2.5c), make sure you have the selections you want checked for Rumble in the top left (Brake Pedal, Brake Wheel, Gear Upshift, Engine) once you have that, adjust the slider below and check to see if that helps.
Very important, be sure to update the driver, then within the driver, update the firmware. Yeah it was weak before in earlier firmware versions but with later versions it was improved. Now if you are on your feet all day or beat up your feet to the point of no sensitivity, then sure you're not going to feel much. It's really subjective. I will say it assists with wheel lock up and is pretty instant. There was a delay before but seems it was improved with later version of Fanaleds or driver.
On average, meaning not just me but some friends also that tend to be on their feet a lot can feel the rumble through most thick soled cross training shoes.
Hope that helps.