Answer: I suppose the answer to your question is "What is formal in your eyes?" A yew hedge would give a very strong and traditional heavy evergreen statement in a garden, where the Rose of Sharon hedge is deciduous and makes a definite floral statement in mid summer. (It also leafs out late in the spring.) Part of the answer would also have to depend on what else is in your garden as to whether or not this would mesh, but it is your garden and if this is what you think would be the thing to have in it, by all means try it!
It can be pruned into a hedge of sorts, although it will not be thick all the way to the ground and will not be as crisply symmetrical as a yew hedge would be. Prune it as needed for shaping in late winter or very early spring before the buds swell. It blooms on the current year's growth so pruning in winter will not hamper flowering. You could also tidy it up after flowering if need be.
These are actually rather large plants, so for long term health I would probably put them about three feet apart.
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