Accepted wisdom is simply that third-parties are at a slight disadvantage because they're reverse-engineering the mount system. If the manufacturer introduces a change to the mount (or more specifically, the electronic signals communicated between the body and the lens), they can make sure all their old lenses work with it, but they don't have to make sure third-party offerings do the same. There are known cases where third-party lenses have had to be rechipped to work with new digital bodies. Typically, third-party manufacturers will offer to do the conversion for free for specific lenses for a limited time, but there's never a guarantee they will offer it for your specific situation.
OTOH, third-party lenses will also fill in holes in a manufacturer's lineup, have decent-to-great optics, and better price tags, so there's definitely a tradeoff to taking the risk.
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