If the flex is large enough to handle the Cfm and its installed right, I don't see a problem with it.
I've had contractors come to me after I bid a job that specs out hard pipe with 6 ft of flex. They ask what we can do to trim the bid. It's always switching to all flex. We've never had a problem with it.
Also, I've done hundreds of what we call "burnouts" for insurance companies. Always bid flex. I know a lot of pros hate it, but its everywhere.
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