see this is the problem with online reviews in this sort of format.
you mention that this is very reliable, but my own experience running 2 of them for over a year and plenty of anecdotes from other users is you have at best a 50% success ratio.
it gets worse over time too with the firing pin blunting and straight up not having enough force to puncture the cartridge, this starts with light strikes meaning the detonation time goes from seconds to minutes.
and that's before we talk about the proprietory "top hat" seal which rips almost immediately and means what gas does come out of the cartridge just goes straight out the firing pin rather than bursting the shell.
it's well documented on various forums these issues, some users cured things like the light strike issue by removing the spoon and letting the hammer rest on the firing pin, whacking it off a hard surface (in a similar style to japanese ww2 grenades) gets a reliable puncture at the risk of it going off in your hands.
there's also the issue of safety, a bad enough knock on a good grenade can be enough to cause a slow puncture which you won't immediately realise is happening until it explodes in your hand, which as you mention bloody hurts.
even out of the box (although they may have fixed this with the newer versions) the timing was somewhat inconsistent and generally too long (well above the 3-5 seconds that would notmally be practical for a timed grenade)
it's a fantastic concept for a grenade, especially for places where conventional pyro/bfg's are a bit more restricted. but the basic design and manufacture just isn't up to the long term and there are a lot of issues that become very apparent if you use them for more than a couple of games.