That concern is quite valid.  Oil flushes from yesteryear gave the category a bad name for that very reason.  However, we took that issue into account when we formulated Berryman Oil Change Flush (part #1216).  The product does not use any of the harsh solvents that the old-school flushes (and some of the current ones!) have used like acetone, toluene, xylene, or MEK.   Instead, we use the detergents and dispersants found in engine oils to do the heavy lifting.  This prevents chunks of sludge from dislodging and clogging oil screens and passages.  Rather, it attacks the surface of the buildup and dissolves it a layer at a time.  We like to think of it as cleaning by way of the onion method.  For this reason, if an engine has significant buildup, it might take treating a few consecutive oil changes with 1216 to clean up ALL the sludge.