But rengar's warning doesn't ruin stealth?
Your logic there is absolutely flawed. Twitch can stealth very very very often while repeating the burst. Rengar can do it every 40 seconds at the most. Twitch can kill a whole team with his, rengar generally just kills one.
You tell me, what weird with that picture.
The fact that you are HIGHLY twisting kits?
Rengar's max combo (damage wise; he could choose for the Empowered E for more lockdown instead) does 3 autoattacks + 630 + 120% AD +170 (80% AP) + 250 (70% bonus AD). In addition, Rengar would have +10% AD (first empowered Q), 30 armor +mr (with an extra 15 for every champion + large monster hit), and a 80% slow.
Twitch's is 6 autoattacks + 290 (120%AP)(150%Bonus AD). During this, Twitch would have 70% Attackspeed, 36 AD, and 300 bonus attack range.
So no, Twitch does not have higher burst then Rengar, and you should feel bad for thinking so.
In addition, if Twitch is sneaking up on a team, he's dead due to being an incredibly squishy adc who just blew his escape/engage.