Yes, there is a fair amount of lumbar stress, but most of it occurs when racking the clean, and is compressive in nature. The spinal column is much more suited to handling compressive force than the shearing force it is exposed to when performing a heavy deadlift, assuming that the torso would be inclined far more with respect to the platform.
If I had to compare them, I would state the following:
1. Clean: Most compressive force, least shearing force.
2. Deadlift, least compressive force, most shearing force (while the weight is significantly greater, the spine is not subject to as much loading due to the much slower speed of the weight, and hence much lower force).
3. The snatch would have more shearing force than the clean, but less than the deadlift. It would have less compressive force than the clean, but more than the deadlift. The lift is moving faster than either the clean or the dead, but the weight is significantly lower than the dead, and a bit lower than the clean.
I have not seen many lower back injuries with any sort of pulling lift, usually when squatting, as some idiot lets the bar get too far out in front of them (I am guilty of this sometimes, but then I never claimed to be all that bright).
The most common injuries in OL'ing are usually knee and shoulder injuries. Poor foot placement when receiving and racking can lead to knee problems, and poor technique when receiving the snatch can cause a great deal of problems, specifically when one is struggling to hold onto a snatch that is too far behind the lifter, and should just be dumped.
The most common injuries, in my experience, when doing any sort of pulls, are bicep and elbow injuries. People forget to keep the arms straight or try to jerk the bar from the deck, and wind up tearing a bicep.
The lifts are not that hard to learn, if you have a decent coach. Start slow and concentrate on technique, you have the rest of your life to add weight. The problem lies in the fact the the process of attaining sports mastery (PASM - Soviet term but very useful) takes many years. According to many coaches, it takes at least seven years to properlly develop OL'ing skills.