The leg press can definetly be hard on the spine if you flex your back toward the bottom of the movement which is common.
With proper squat form and progression it should make you back stronger.
check out this articale it's required reading.
My back has gotten so much stronger since i started box squatting. In the fall I did 300 pound good mornings with the bar up on my shoulders. Which may not be much. But two years earlier I was afraid to use more than 75 pounds.
And you may say thats anecdotal but all this anecdotal shit is backed by experts in the field. Its no conincidence.
Furthermore many anecdotes arent exclusive to a few people you can see them in every gym across the country.
We see people that can leg press 800 pounds NO LIE. 800. But struggle with a 135 pound squat.
Guys that leg press 800 pounds and have to use a belt to strain out stiff leg deadlifts with 175 pounds.
People that think of every excuse in the book not to squat or deadlift. Even though they assure their form is right, it usually isnt. Their knees are the first thing to flex they flex very little at the hip, pathetic.
General population can get away with leg presses but if you want the edge in athletics the squat is the way to go.