Hy Mimimadi,
Well your question sounds simple, but it is not.
Has too many specifications to be answered. I will do my best as far as my knowledge allows.
I would like to start saying that there is nothing that makes diving at a 100ft less risky than a 60ft.
I would also like to say that with the correct training and knowledge and of course common sense i would say that it is not more risky, but there are a few more things to be considered and taken care of.
Lets take a quick view on what it should be said...
Air consumption:
Well obviously, due to the depth,for the same period of time, you are gonna be using more air at a depth of 100ft than at a depth of 60... 40% more. We could say that is more likely to run into an out of air situation. But this can be avoided very easy by checking often your air gauge and briefing with your buddy a pressure where is time to go back up.
NDL:
One limit on your dive that you start considering seriously when diving deep is the No Deco Limit. Not only at this particular dive but on the consecutive dives of the day even though they might be shallow. So if you don´t take care of that you might run into deco, witch is something a Recreational diver should (I say should because is not uncommon) never do. So as we learn in the OWD course running into deco has its dangers.
Narcosis:
You´ll experience narcosis if you cross the 30m depth. If you never experience it you might end up in troubles, so you better choose an experienced diver that has plenty of deep dives otherwise the two of you could end up doing something really stupid and dangerouse
Oxigen Toxicity
Even though we are talking about depths that are far away from this risk, at the moment you decide to dive Nitrox, the Oxigen toxicity becomes a real Risk.
Buoyancy
I add buoyancy in this list even though i consider it vital for any dive at any depth. But, multiple accidents happen, including in the Advanced training you mentioned, due to massive over-weighting.
You can read one of my articles about this problem:
The bad habit of overweighting
Now I would like to tell you my pint of view.
Its all about common sense.
Under my point of view lack of common sense makes diving dangerous at any depth. Under my point of view, a diver that knows and understand the risks and dives under the basic rules at a depth of 30m is more safe that a diver that doesn´t know what i doing at 18m.
When something goes wrong at a depth, one thing that will decide how it will end up is your experience and capacity to remain calm and think. But the other thing that will make a difference is how have you been managing your dive.
If, for example you separate from your buddy because you want to take a nice picture from a clown fish, and never nothing happens, well is ok. The day you have a free flow regulator or you run out of air, you´ll regret to have taken that decision.
I would say that Depth (Recreational Diving depth) requires more procedures, more understanding and more common sense (more training if you rather). But if you follow that i would say it is not more risky. Some things can go wrong, and if they do, they can be much worst at a depth, but we train the divers to solve those situations (at least i do) and on top of that, we train them to avoid them.
So i am not sure if i clarify your question, but like i say always, in diving you will rarely find a unique answer for a topic. Wait a bit and you will see how many different answers and oppinions (most of them for sure valids) you´ll get from this post.
Happy to help, please quote me it would make me really happy
Happy Bubbles
Gery #scubalegnds
http://www.scubalegends.wordpress.com
http://www.facebook.com/scubalegends