The bearing has no idea what type of bike it is fitted to....
When on a mountain bike, the bearings generally will be exposed to more dirt and water. More often those bike tend to get washed down more, and with higher pressure. So in that the bearings that would be fitted to mountain bikes may have bearings with better seals or covers.
I ordered six for my Zipp 303S wheelset yesterday. The freehub body bearings have given up thrice (counting twice on my 303 Firecrest which share the same 176/177 body) and the rear wheel bearing twice during wet, unsupported ultracycling racing. Aiming for 4.000 km racing and need to be able to ride the whole distance.
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trade off appears to be no need to lube at the cost of higher friction, though unless you're doing tt it'll make no practical difference