Are there any down sides to flushing?
Only if you over-do it mid cycle.
Some growers panic and flush too heavily, for too long. Make this mistake and you can go the other way and underfeed plants!
If you’ve overfed plants and the leaves are just starting to curl over or burn at the tips, go easy! Try using a half strength nutrient solution instead of applying lashings of water.
Doing it slowly like this causes less osmotic stress.
The best method, of course, is not to overfeed plants in the first place!
If the EC rises in a recirculating hydro system, plants are using less nutrients and more water. Reduce your nutrient strength before plants get overfed.
In a hand-watered or drip system you can check the EC of run-off. If it’s higher than normal, either get more run-off when you feed or reduce the nutrient solution EC.