Is the SeaSonic S12II 620 Bronze 620W good?

Most everybody makes some good ones and some bad ones and several in between. Seasonic is one of the most consistently reliable tho. Some make just bad ones :).

Seasonic makes a lot of PSUs for other folks, many in accordance with the vendors spec which may not be all that hot. Those lower end platforms may never me marketed under Seasonic's own brand name.