I reckon that, in the 40 plus years I've been riding road and racing bikes, this must be the most discussed topic!

Until the governing bodies of the sport; ACU, RAC, ect, get to grips with proper testing and publish the results we'll never really know which is best for safety. That'll not happen though, as the smaller companies will fall behind and leave the big three to rule the roost and prices will soar.

Comfortwise I reckon that only one person can be the judge of that, and that's the wearer.
As for being 'fit for purpose', I reckon that riders need what riders need; some like a full face, some an open face, some a flip front, and every rider is different but not all manufacturers cater for all riders, hence riders wearing what they perhaps judge to be a less 'safe' helmet of 'cheaper' make.

Every rider has different requirements of a helmet, apart from it's efficiency when we really need it; in an accident, but safety also means having the 'right' helmet for you to start with.
For instance I wear glasses and I'm a bit claustrophobic. Not a problem? Well, ride facing into the low winter sun here in the North, especially after a rain shower, and with a visor (or goggles) and glasses any imperfection or smudge on the visor or lens makes it impossible to see.
Flip the visor up (or take the goggles off) and the wind swirling behind my glasses makes my eyes water badly, and I end up pulling in to clear my vision.
I have tried lots of full face helmets over the years, but the only one I've been happy to wear for any longer than a few minutes was a Griffin, which tells you how long ago that was. If I could buy one now I'd happily pay what 'A' charge for their top model.
That helmet had a good distance between my mouth and the front chinbar, and I could ride for days with no problems at all, but 'P' and 'A', not to mention both 'S' makes, have been tried and all make me feel too constricted. It's not just 'discomfort', it verges on real distress, and is a failing I know, but we all have our fears, don't we?
Motocross helmets are fine, with lots of room and a huge area of vision, so I feel fine with them, and the peak can be tilted to keep out the sun from my glasses. But, at cruising speeds on the BMW wind can be a problem with such a big peak.
So, I bought an open face B*ll Tourlite, a pair of O*kl*y L frame goggles and a leather face mask cover the gap between chin and jacket collar and I'm fine! OK, in a crash the lack of frontal protection may result in some facial damage, but if I'm comfortable and concentrating I'm less likely to suffer an accident than if I'm worried all the time and can't see properly!
One company (A) makes what is almost ideal for me; but the peak is still too big to be comfy at speed, while yet another (who also make bikes, German ones) has a good sized peak, but too constrictive a chinguard.

So, my open face Be** is not as expensive as a full face from the latest big manufacturer currently in the public image with WSB riders wearing their latest design, nor as safe as it offers less facial protection, but it's right for me.