Jayce ADC and JG

ADC - bad lategame, because you can't safely take towers with that range unlike all the popular ADCs. Probably works with all-in champions that dive towers; in the laning you want lockdown support, so you can just burst down the enemy ADC. His damage is not great before lvl 4 and you will get outtraded early. Jungle - he has really low base damage and is squishy without having good ganks. I can't see him being a playable jungler (I've tried playing him once and the first clear is a nightmare). The games being faster and the armor pen items changes were nerfs to him. There are just better champions for poke, all-in or sieging than him atm... He is not rewarding for a high skill cap champ and has mana problems, so you have to go tear, if you want to spam with E+ Q.

he's a top/midlane AD assassin/bruiser...

He can't ADC because he has low range, awkward trading in a duo lane, and scales better with level than gold.

In theory, he can jungle if he survives his first clear with kiting/defensive runes. He ganks well for any laner that enables him to land his full combo, which gets really strong really fast.

Almost certainly better top or mid, though, wherever you can find an advantageous matchup.