Jace is one of those champions that's really strong in 1v1 or in poke fights, but his lack of a reliable stun, escape, or all-in ult makes him worse when junglers are involved.
He's no Fiora, but he's worse than your average top champ in this regard and it's his most easily exploited weakness.
His numbers are reaaaaally ridiculous. I did a double-take when I first saw em. I'm talking max level here because that's generally the baseline.
To the Skies! is actually kind of balanced. 50% slow is quite good but nearly his only form of CC. It has solid damage and a 1.0 AD ratio, but is really not that dominant. Just a bread-and-butter gapcloser.
Shock Blast has 280 + 1.2 AD ratio, 1050 range unaccelerated, close to 400 + 1.7 when you gate it. Not to mention that when gated it's so goddamn fast that you're toast if you're not hiding behind your minions.
Lightning Field does 380 magic damage +1.0 AP - damage over time, but quite damn significant. Also, passive mana leech, because why not!
Hyper Charge is essentially a 3.9 total AD ratio ability that scales with crits, on-hit, lifesteal, whathaveyou on a six second cooldown.
Thundering Blow is 20% maximum health. +1.0 AD. (Also, knockback.) Because we heard that Jayce's damage is apparently too low.
Accel Gate? Team speed buff of 50% that fades over time. That's good, don't you knock it. It also makes his Shock Blast into the fury of an angry god, but you already knew that.
Transform? Hahahahahahahaha. The free stats aren't that bad - the Cannon resistances shred is respectable when maxed, the free AR/MR in Hammer Stance is cool - but does it ever pale in comparison to the entire rest of his kit.
I don't understand why the hell most Jayces I see max E second (or put points into R at all). Accel Gate gains little from levels and Thundering Blow also seems to scale better when it's leveled late. By contrast, you could get the Electrocutinator and God's Own Pewpewpew from annoying to true terror level.
Total ratios: 1.7 + 1.0 + 3.9 + 1.0 = 7.6 DAMAGE PER POINT OF BONUS AD (and 1.0 per point of bonus AP). 920 + 20% max health base damage. This does not include regular hammer whacks/cannon shots and 25% AR/MR shred, which it should, seeing how I factored in R's hammer transform magic damage already. A properly fed AD carry can match this damage in about 10 autoattacks, though, and that's probably your point of reference. Jayce isn't all that bursty.
Also, he should probably build pretty AD-heavy, while his base damage is good his ratios are too insane to not use them. Possibly a Manamune - he charges it really fast - and some CDR. He might want at least some AS as well, I guess, no matter how his kit looks. A bit of sustained damage would do him good. Although, perhaps a CDR-capped Jayce can cast random stuff often enough to not need AS at all.