Straight parallelogram derailleurs do shift OK, most of the time, just a bit slower shifting and a bit less precise that offset pivot (Like Simplex SLJ6600) and slant/offset pivot (Like Suntour Cyclone Mk II). They usually require more overshift action at the lever to shift and more "trimming" after shifting. Their design simplicity though, seems to give them more reliability and ruggedness. That's why many pro and amateur racers still loved using straight parallelogram derailleurs like the Campy NR and SR right up until the beginning of the mid 80's, even though most other makers moved on to the later offset and slant/offset parallelogram designs.
As already noted, shifting performance on straight parallelogram RDs do benefit a lot from proper, careful adjustment. While offset and slant/offset parallelogram RDs are not as sensitive to this.