Curved bars are usually only used when it's absolutely necessary to get a little bit of clearance between the center of the spring bar and the case of the watch. You'll almost always be using straight spring bars.

 

^^^^

This.

 

To expand a bit, curved spring bars are generally reserved for when the manufacturer wants the strap to be snug up against the case...no gap. That means the lug holes are drilled very close to the case. The downside primarily is, those holes won't readily support a bracelet, as I've seen few curved-end metal bracelets. Similarly, options in silicone or rubber are few and far between.

The Ming 17.01 is this way...but for the 17.03 and IIRC the 19 series, they actually drilled 2 sets of holes, so a standard-end strap can also be used.

 

As mentioned, not common place and usually only reserved for getting strap clearance on a tight lug, Rolex Datejust is a prime example, using curved spring bars will give that clearance to stop the strap rubbing against the case, leather against metal, will only be one winner !

 

I have the impression that curved springbars won't be as strong as straight ones?

 

Interesting question. I know some people who gets curved spring bars by bending straight ones on a spoon. There are however proper bending tools, but I would still be reluctant to try one. How it will affect its strength i wouldn't have no idea lol.

I get my curved spring bars from JLC directly as they make them. Ofrei also sells swiss made curved ones.

 

I doubt strength would be compromised. However, it seems likely they'd be a bit more likely to pop out, just based on the geometry of the hole and the bar. That said, having it pop out would still require an unusual, sharp tug from the strap. On a two-piece strap...seems implausible. On a single-pass or NATO strap? I'd be more worried. Perhaps wrongly, to be sure, but more worried.