Ours typically have a bit in the middle that's usually grass behind a curb, then a lower raised bit with a bevelled curb that's all concrete surrounding that, or else just an area around the center-grassy-bit that's even with the rest of the pavement but marked off with "don't drive here" stripes. The ordinary lanes are outside both.

I assume those are affordances so buses (say, school buses), trucks towing trailers, and stuff like that, can actually use the roundabouts. They'd be damn tricky for them to navigate, otherwise.

Kinda off topic, but this reminds me of a racing game I played which had a track that featured a roundabout with a bump just perfect for Dukes of Hazarding it.

So ideally the roundabout also doesn't encourage sweet jumps. (Unless there's a very well-designed designated landing spot that prevents damage to the vehicle. Probably tax dollars can be spent on better things.)

What if they put a flexible bollard in the center?

All anecdotal but hey: The roundabouts I've seen fail (we have many) are those with not enough and too much obstruction. The versions with some form of gradual grace (curved edges around a center platform or lots of grass and a bump) tend to work best. One of the worst ones was a very large one with a meter high round wall as center barrier which caused lots of damage to those who made a mistake. The too small ones are just intersections with minor obstacles to navigate.

Tall stuff impedes vision. The gental slope around tends to be accommodating for tractor trailers. Short stuff, even just a regular curb inside of the sloped area should be enough to stop cars from driving through it. Some short flowers or plants can help draw attention.

You want to impede vision across so people focus on what is important for their merge.

That's only potentially true with very large circles. Small round abouts are so small that you want to be able to see obstacles before you even get to the point of preparing merging.