If you are using proper form, no leg exercise is bad for your lower back or knees. Anyone who hurts themselves doing legs is ego lifting.
The leg press certainly has it's place in a leg workout but nothing will ever replace the squat when it comes to overall leg development and size. It is truly the best exercise you can do.
Not a smith machine squat, not a hack squat, not a front squat, a real squat where the bar is resting on your traps.
Master the squat and focus on increasing the amount of weight you can squat and I PROMISE your legs will go from tooth picks to tree trunks.
Most people do not get the leg development they want because they don't understand the anatomy of muscles in the legs. The legs are an endurance muscle. They have a large composition of slow twitch muscle fibres. Most people only train in a 8-12 rep range. The problem with that is it won't stimulate the slow twitch muscle fibres. In order to fatigue the slow twitch muscle fibres, you need to train in a rep range of 20-100.
Use a variety of exercises to fully develop the legs but make sure to add in some 'burn out' sets at the end of your workout in a rep range of 20-100 to stimulate the slow twitch muscle fibre. You will feel a burn like you have never experienced before.
Think about it. Look at the legs of a cyclist. They don't squat or lift weights at all but they always have extremely impressive leg development. That's from all the high rep work they constantly do when cycling.
Similar to legs - forearms are also comprised of a lot of slow twitch muscle fibres. Make sure to include some 20-100 rep sets into your forearm training as well.