I see people from time to time bring up folding levels and how rigid levels become a snag point, but I don't really understand the argument, or at least not with the new level offerings companies are producing now. A quality scope mounted spirit vial level from the better brands have thought about that and designed around it. There are a few companies out there making nice levels. Accuracy 1st, flatline, MK Machining (shameless plug).
My [small] issue with the holding levers is there will almost certainly always be a little bit of slop, meaning they will not be AS accurate and likely not as durable if something were to happen. Now I understand that the little amount of "slop" does not result in much rifle Cant and most consumers do not need a level that accuracy. The folding mechanism does add cost though and to me the added cost for feature is not worth it over other options out there.
The placement of the vial on a well thought out model should not stick out past the parallax knob and should be well under the elevation turret making the bounding box of the rifle no larger. Meaning the level should not be anymore of a snag point than the knobs of your scope.
Here are a couple photos of our newer XLP level mounted to two different optics.
This first one is mounted on a Swarovski Z8i. It is a little tricky to tell in the photo, but the level head is flush with the parallax knob. I don't have a photo of this, but we used a Vortex LHT optic to optimize our design to make sure you can still see the vial with your non-dominant eye while still in the eyebox/shooting position. The LHT models have pretty much the smallest parallax knobs of the commonly used higher end hunting optics that I'm aware of. (I'm sure someone out there makes one smaller)
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Second picture is on a Zeiss LRP S5. The S5 is certainly more on the tactical end with large turrets, but the photo will help give you a perspective of scale. Really large parallax knob, and you can still see the complete vial from the shooting position.
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