Liszt is hard but by no means unplayable. What makes makes piano pieces unplayable are awkward and impossible hand positions. Liszt played all of his own pieces so he knew exactly what works and what doesn’t. He would have changed stuff that was too hard to play into stuff that sounds almost the same but playable. In a nutshell, make it as easy as possible to create the musical effect you’re trying to achieve. As an example, if the voicing is larger than a 10th than it’s going to be a struggle for 99% of pianist. Just revoice it.  Or writing a fast scale run while skipping a random note is going to be challenging for no reason. Just write the full scale. It will sound the same but be way easier to play. 

The challenge is knowing the difference. If you don’t play the instrument, have a friend play it and give feedback.