only a problem if the water level is lower than the tubes.
i have several customers with this or other similar sealers in the system due to block and/or head cracks, and not had any problems with blocking. we circuit race, too, so these cars do run races up to an hour long... the shortest races would be 15 mins.
the one time i was involved with a car that did have issues was when a bucks down racer friend had a pinhole in his 14 second drag car's radiator. he had sealer in it, was sealed up and working well and we were tuning on the dyno. it threw a power steering belt, the dyno operator paniced and shut it down halfway through a run. of course it was red hot so it then popped a hose blanking plug off, which then lowered the level of water in the radiator which instantly blocked it!! another used radiator was an easy fix, but it also outlines my personal opinion that if you cant afford to race properly, dont bother!!