Does using pH up/down affect your plants?

In a nutshell, yes.  pH up and down are acid/alkalie.  Both have to be used to calibrate pH typically to 6.0. 

Tap water is pH 7/8 and must be adjusted when mixed with nutrient.  Fortunately most nutrient mixes down and gives pH 6.0 once ready.

The argument for NFT systems can been seen further, because the pH stays solid 6.0 mostly.

WIthin flo-gros, the pebbles make the pH run away to 8.0 or 9.0.  So daily adjustment is required until the pebbles have been suitable used for a while and the ph variance reduces.

pH is critical, maybe not so much as eC strength but your plant will underperform if pH is all wrong.