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Note that the “site_” functions are essentially the same as their counterparts, but work network wide when using WordPress Multisite.

Also of note is that Transients are inherently sped up by caching plugins, where normal Options are not. A memcached plugin, for example, would make WordPress store transient values in fast memory instead of in the database. For this reason, transients should be used to store any data that is expected to expire, or which can expire at any time. Transients should also never be assumed to be in the database, since they may not be stored there at all.

Furthermore, it is possible for the transient to not be available before the expiration time. Much like what is done with caching, your code should have a fall back method to re-generate the data should the transient not be available.

Ryan McCue explained it this way on a ticket: