Additionally, several threads have discussed NOT using any UA content when multiclassing. Is this an official recommendation or merely conventional wisdom?
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It is more of a recommendation, with one big caveat. WotC is putting these out to see what people think about them, but they do not care what you think about them in regards to multiclassing. They have said so repeatedly. They have also repeatedly said that the final version of things may be different than the UA release. So for personal use, go for it. As part of a "playtest", it is very much a no-go. The forums complicate things, because there are people who like to brag/proclaim the sky is falling based on multiclassing UA content. Is that personal use or "playtesting?" So if someone says "you can't multiclass UA stuff", what they usually mean (even if they don't know it) is: as far as playtesting goes this isn't really a problem.