I mean, it's all baseless speculation until we see what the notables/keystones are.
Entire system will live and die by what is there and how viable any of it is. If it's anything close to what the timeless jewels can do (without the insane rng) it'll open some real interesting possibilities
Some of the examples they've shown were pretty interesting. The summoner one was pretty good, you get solid passive nodes(bit of flat life+chaos resist+minion life, so not bad at all although the minion life could be something else) and then a keystone, which obviously is going to be more or less useful depending on build but the summoning one they've shown is very interesting, making minions unkillable for a few seconds after being summoned is a pretty big deal for squishy summons in boss fights(shit like phantasms, animated weapons, SRS or even skeletons with their terrible summoning animations).
But yeah they have a high cost associated, they're directly competing with nodes already on the tree you can reach in the same amount of points. And have to remember you can only slot these in the external jewel sockets, most of which are actually fucking garbage to begin with. For example say I do a Necro. My choices are the socket between templar and witch(natural choice since you path next to it to begin with), but it's 2x 6% elemental dmg before the socket, so you need 3 completely wasted points+jewel just to unlock the stuff. The right side one is "better" with 2x 4% mana but that's not great and you don't naturally path there in the current builds unless going CI. Then you have the templar/marauder one which you get pretty close to/path through if doing a more life build, but that one's 2x 8% armor so it's really pretty bad too, not as bad as elem but almost.
So generally speaking you're going to be wasting 3points to even get started. The other nodes are 4% melee phys/ailments for marauder/duellist, 8% evasion for duellist/ranger and 4% proj dmg for ranger/shadow. The melee/ranged ones are like the elem one in that they don't even do anything for some of the builds that might want them.
One notable thing I've read from Bex on reddit however is that while you will need regrets to unspec them to change stuff, you will only need to regret the notables/keystones type nodes. All the minor nodes in these new jewels can be unspeced for free, so that's nice if you just want to switch stuff around.