@Sergio, the devil is in the dose. If your technique is solid for a deadlift or a swing, you'll be fine. But if you get a little fatigued and keep going, or become distracted and don't brace properly, then the weight becomes heavier than you can handle and you can get hurt.
I look at my various injuries and pains as blessings - they make me more aware than most people of when my form is less than satisfactory.
I strongly recommend you read "Treat Your Own Back" by Robin McKenzie. Mine was give to me by my doctor. It will go a long way towards explaining to you what may have caused your back injury. In my opinion, unless we're talking about something catastrophic like an accident, most lower back injuries don't have a single, simple cause.
To continue what Brett said, I only began deadlifting with my doctor's blessing.
I have found I'm more able to maintain strict form in a deadlift, which is slow and controlled, than I am in a swing. Moving quickly, even with a much lighter weight, is nonetheless something that requires a level of skill that is in many ways higher than moving slowly, even when moving slowly involves more weight.
I hope this is helpful. My story has been told many times here, including in an article on this site from late 2012 or early 2013.
-S-