I like the handlebar height lower than the average road bikes I see in my city. Am wondering if I am putting myself in more risk.
Being seen - the higher you are, the more likely you will be seen over a roof top. Clearly some cars are too high or too low for it to make a difference, but the odd car is at a height the difference might be significant.
Seeing - in an aero position, its harder to see as much as in an upright. It can be done by actively looking around, but its harder - you have to force yourself to do it.
Speed/efficeny - when I go into the aero position, its because I want to go faster or ride more efficiently. My concentration moves towards my riding technique than observing the road. However, it is entirely possible to sit upright and not concentrate on the road (e.g. Focus on music, sending text messages, last nights hot date....etc), so I believe upright is safer with the proviso you are concentrating the same amount on riding.
Manoeuvrability - when on aeros your ability to change speed and directions is restricted. If aero position did not compromise agility, XC MTBers would have drop bars.
So overall, I would suggest that Aero position is less safe than upright. Quantifying how much difference it made would be harder...
Yes - visibility is everything for both the rider and the surrounding things.
Here's an example of a road bike in traffic.
The effect is exaggerated because camera is on handlebars, but even at head height I didn't see her till the camera did. You can see my body position by the shadow on the left side. Its New Zealand so we go on the left side of the road, and that's a "marked cycle lane" The road there is a three lane each way, with a posted speed limit of 50 km/h. However its 0830 traffic. I was doing "low 30s" according to strava.
So had I been on a flat bar my head would have been 10cm (4 inches) higher. Whether the obstacle looked anyway, I don't know.
Downside from being on a MTB or hybrid, your handlebars are much wider, so it far easier to whack a side mirror or a stanchion/bollard or even another cyclist.
Also, you're slower on an upright than a road bike.