Sea buckthorn refers to the plant hippophae rhamnoides, and its usage as a supplement can refer to either the leaves of the plant or the berries that it bears; additionally, the berries can be supplemented as either a dry powder or as an oil derived from the berries. All parts of the plant appear to be bioactive.
This plant appears to be a good source of flavonoids, mostly those structurally related to quercetin and kaempferol. It also has procyanidin (chains of catechin molecules) content like some other plants, with some epigallocatechin and gallocatechin as well (half of the green tea catechins). While Hippophae rhamnoides doesn't have any well researched unique properties (The hippophin molecules are not very well researched by themselves), it appears to be a good collective source of common flavonoid compounds.
Historical usage of this plant has been for cardiovascular and blood health, and it does appear to protect the heart itself in rats and confers anti-platelet effects following higher recommended doses of the supplement in otherwise healthy persons. Some other benefits of this plant, while not completely unique to the plant, include accelerated wound healing and improved skin quality following oral ingestion and some basic neuroprotective properties.
Hippophae rhamnoides is effective at helping with many common health goals that other flavonoids are effective at, and while it seems to have a large base in traditional medicine no highly unique properties or molecules have been detected with this plant yet. Currently, sea buckthorn can be said to be healthy but there is no one reason to supplement with this plant over others that are more effective.