I've never had it actually had it fill-up, to the extent it overflows, even if I leave it for a couple of days. But then I'm not using FB or Hangouts that put multiple notifications up for everything that happens. The FB app showing ten notification icons, that's poor design on the part of FB IMO. If your Facebook is busy with status updates from all your friends, you could be clearing icons every few minutes. What I'm using seems to be well behaved, one notification per app, as I think it should be. If there is a notification from say WeChat, I can open it and see what's in there, also if I open the notification draw it says something like "2 messages from 2 people.", but there's only one notification icon.
I've never had Facebook put ten icons in my status bar, though I do know it can put multiple icons there, but not for the same type of content. If you have an event invite, then it has a different icon for that. Same for friend request and Comments, etc. The icons are completely different, and the way they've implemented makes since given how so many of their users use their service. It also allows you to see without even pulling down the notification bar if the Notification is something worth dominating your attention. IIRC, it also keeps different categories of Notifications separate from each other in the Notification Shade, allowing you to disregard or dismiss the uninteresting ones and go straight to the ones of note. From a device usage point of view, that is very efficient.
Unless I have you have a privacy setting to specifically not show Avatars/Names/Message Previews toggled on (like you can with the Samsung Messaging App), "2 Messages from 2 People" is unacceptable for a Messaging app, and I wouldn't use something like that as it would result in too much wasted time especially if you have a fair number of people who communicate with you using that service.
That's like an Email Client saying "2 Emails from 2 People" and forcing you to open the client only to find out it's a Newsletter from DirecTV when you switched to DISH 3 months ago. I don't want to have to go into an app just cause a couple people in a group chat replied "Lol" to a joke that I told them 2 hours ago that they just read. I want to see it and disregard it without the extra actions.
Notifications give you these different icons and different information so that you can optimize your usage of the device. This allows you to look at the Notification and completely disregard it if you want, and perhaps skip uninteresting notifications from the same app and go straight to those of note.
That's all good.
The part where Android's notification system breaks down is when you have Multiple Notifications from some apps, like Gmail, the Email App, or some OEM stock apps like the Samsung SMS/MMS client.
In that case, you can Expand the Notification, but the system does not allow you to go directly to the subject of a specific notification, so usage is not optimally efficient in this use case. You have to click the entire notification which thing brings you to the "Home screen" of the app, and from there you must click on the item that interests you. This is much slower than iOS' way of simply expanding the group and clicking or swiping (forget which it is) on whichever one interests you and going straight to it - without regard for those other notifications, and bypassing the extra step by not dumping you at the home screen of the app - but at that specific item in the app (similar to a notification for one email from Gmail on Android goes straight to the email and not to the Inbox; which is what happens when you have multiple notifications from Gmail in your notification bar).
From the Android L previews, the look of the Notifications has changed (in some cases for the better, like the Incoming call Notification is extremely well-done and makes complete sense). However, the actual implementation of the Expandable Notifications seems identical to what we have in Kit Kat, and for people with busy devices, this makes them do too much work to keep their notifications completely cleared (sort of Inbox Zero) on Android, when they could easily disregard the uninteresting ones on other platforms.
