Why is WMM so broken?

When starting to use my BT Homehub 5A with OpenWRT, I've realised the speeds were too slow on 802.11n cards (capped at 54mbps) but I later found out that the culprit was the "WMM Mode" being disabled on the 802.11n chip (Atheros AR9287). After enabling it, all was fine and my devices were using the full speed (72mbps on 1x1 and 300mbps on 2x2) but when using my other laptop (with Broadcom 4313GN chip), the Wifi keeps crashing after a while. It's only when I disable WMM on either the laptop or the router that things stay stable.

So my question is: Why is WMM so poorly supported on Broadcom chips? Is it a driver or firmware issue? I've seen other people having the same issue even after changing OSs (Win 7, Win 8...) or drivers. If i recall correctly, other routers don't exhibit this problem, albeit having Broadcom wifi cards.

I'm not asking for the OpenWRT community to fix broadcom chipsets. I'm asking why WMM isn't as good as in other routers?