Yeah one of the things that drives me a bit crazy about the phone. Many folks have voiced their opinion about it at the ASUS Zenfone forum but I think we're stuck with it being a weak light. Plus it only has 3 colors as well.
Maybe its certain models. My Z2 the cheapie version the notification light is every bit as bright as my Nexus was. In the dark it is like a beacon. In the daytime if it blinks and I look at the phone it is quite visible.
I'm also disappointed by the LED. There's only yellow (notification), orange (battery low), and green (fully charged). For my two previous Nexus (4 & 5) I used LightFlow and I loved it. I put a different light and flash rate for different types of notification. Now it's so bland.