It's not only at higher frequencies that the squarewave can cause problems. It can also wreck havoc at lower frequencies like I demonstrated in the attached video where I look at a 1MHz amplifier.
Then there is the unleveled drive, lack of programmable attenuators....... Still for less than $200, you can get the original NanoVNA along with the LiteVNA which would cover a lot of experiments. As you suggest, you just need to be aware of their shortcomings when setting up a measurement.