there's not really a healthy way to balance rengar as an assassin if they want to keep his identity in tact.
either he loses the super long stealth in exchange for assassin playstyle
or he keeps it and becomes a bruiser/diver with a way out.
no one wants rengar who can burst from stealth and has a way out. i mean we did have that. we had him when his CC cleanse made him immune to cc shortly after. it was cancer.
hes still potentially cancer because of his free ad making bruiser so good on him, but calling him a diver is better to admit at this point.
they cannot balance a full one shot from stealth mechanic even if its delayed to 2 seconds for burst. its just not healthy. the same reason eve was prioritized over other outdated champions like cho/malph/rammus etc for an update. because perma stealth or very long stealth is very broken in league. the only champion who is remotely balanced with it is twitch and thats because he is a marksman not an assassin.