Assassins was produced by the not-for-profit Roundabout Theatre Company and thus was not a hit or flop or in anybody's loss column -- that only applies to commercial shows. It fulfilled its limited engagement and even extended, so it likely repaid the company's budget expenditure and that's all that Roundabout could have hoped for it to do. That it didn't transfer to a commercial venue is neither here nor there -- no one, including Roundabout or Sondheim himself expected a show about presidential assassins to have widespread appeal or run more than a few months.
What's Saturday? If you're referring to Saturday Night, that played OFF-Broadway and also was under the auspices of a not-for profit theatre, Second Stage. Just like Assassins, it fulfilled it's scheduled limited engagement, and not being a commercial venture it cannot be called a hit or flop.
It's well-known that A Little Night Music (601 performances -- 17 month run) and Company (705 performances -- 20 month run) recouped and turned a profit. The vast majority of shows that ran well over a year in those days -- with a few exceptions -- recouped.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney