Bardiche: Agree with Nephrite on that.... for example, on Floor 4, there was FFX-2 Thief Rikku. She's got high-speed and Stop, a rarely blocked status. She's not actually that scary due to her support being meh and only knocking out one person, but unless almost your entire team is status-immune, she just wins the fight nearly immediately with a brutally fast MT Stop.
Jo'ou: I still think you're making a mountain out of a molehill. You think that Tension is underpowered. Fine. There are a few sealstones currently that also are pretty tricky to use, but eh, so it goes.
As for interpretation issues, uh, I wouldn't even know why you'd think that'd be hard. In fact, the Status Protection Law already uses stat fiddling (and the old Firefly kinda did?), so making an issue out of this is really a more general issue. The sealstone is clear: it's a 10% flat boost. If you normally would peg an ARPG character at 100% speed, they're now 110%. If for some reason you'd peg that ARPG character as faster or slower, take whatever it normally is and add 10%. As for SRPGs, again, it's obvious - add a 10% boost to whatever you'd normally use!
(To go into it in a bit more detail... okay, I'm assuming that what you're getting at is that not all game stats are created equal? Except that the assumption with all these stat-fiddling effects is that they happen AFTER the game system is done. In other words, if somebody declared that their speed should be unaffected by Status Symbol because the speed stat in their game is nearly irrelevant, they'd get a trout slap. Or for a more concrete example, a 10% boost in a FE character's defense, literally, is worth more than a 10% reduction in damage... but not for any of these sealstones, which simplify things by taking the end result and pushing it up/down. Which is not to say that stat penalties can't be cheesed - plenty of effects are along the lines of a straight-up "70% chance of paralysis" or Tidus's Initiative that really do ignore the caster's stats - but not things like speed, damage, defense, and so on.)
And giving examples of characters that don't work well with the Sealstone... uh, yeah, they don't. You probably shouldn't stick Resourceless on Lilka either. My assumption is more that a team would take slightly below-average speed and above to use with this - a ton of duelers in the DL are "average" speed, and such a team would solidly out-speed them now. Obviously there should be one or two fast characters as well, and they won't mind being even faster.
As for Death, the idea is very flavor based: it's trying to model damn permadeaths from Fire Emblem style games. It isn't an idea that should be encouraged very much due to the path splits it can encourage, but it seems reasonable to have around. I don't see it as too bad a function-flavor disconnect, either: you're getting "half a character," hence the discount. (Actually, Tension was very flavor-based in its start as well, as plenty of RPGs have the obligatory "timed escape from exploding castle/house/space station/etc.," so I was thinking what that would imply as a Sealstone.)