I feel the thought behind this question is wrong.
Some recumbents are lower than upright bikes.
But on my recumbents my head is on the same level as road (racer) bike riders with their hands on the hoods and higher than their head when they are on the drops.
And my bike is a mid level, there are many recumbent bikes which are higher than mine.
My field of vision is very open, I can easily see all in front of me and to my sides, while they can see the ground just before or under their front wheel.
If car drivers can not see me, can they see those road bike riders? (And a valid question in the Netherlands, can they see children on their bikes?)
These days many cars are so high that even on a tall sit up and beg Dutch bike I can not look over the top of them, and nor can the drivers of other cars see over them to see me. Seeing through cars is not easy either. And ever been able to see through the corner of a building? I have had three days in a row I stopped at a corner (where I have right of way) due to traffic approaching that corner too close to the parked cars and buildings along the road. (Better to stop when you hear a car coming.) Whatever the bike I had been on, they would not have seen me in time to stop for me.
So if someone wants to ride a bike, any bike, teach them to be seen and to be careful when they can not be seen.
But the type of bike, recumbent or sit up, is not making that much of a difference, until you get to the very low ones.
Low racers and low trikes (often the tadpoles) are below the nose of the normal cars and have to take even more care to be seen. Adjusting speed might be part of the solution. I personally do not want to ride those in traffic, but that may change when I have tried it for real. Never having used a bike that low I can not tell how cars react.
Added:
I now ride a velomobile and those are low. It helps that it is a big blob of colour, yellow in my case, and with good reflection all around. But I never feel 'unseen' where I would be seen on a higher bike, except when the green on the verges is higher than me, in which case I come to a stop at any crossing path for other traffic, including bikes.
My velomobile, a Quest, with standard sit-up bikes behind it.