I know aero bars are considered unsafe for peloton, does this extend to disc brakes whose rotors can be hot and sharp? I'm not talking about races, just casual training rides: is it reasonably safe for others in the group?
I have never ridden in a peloton but I would say they should be not more unsafe than the chain blades that are already on your bike.
While riding, the only parts of your neighbour's bike that you would be able to hit should be the handle bar and maybe their pedals. Everything else is far less outstanding than those two parts and you should collide with your neighbours body before coming even close to say their disc brakes.
So the only real chance to hit someone else's brake rotors might be in case of a crash where you fall on their bike or their bike falls on you. In this case, there are a lot of parts on a bike that could give some bad injuries, for example the chain blades. And you will also hit the ground, so chances are high that you get injured by the fall as well. Adding disc brake rotors (which even are somewhat "shielded" by the wheel itself) might not add significant injury risk in this case. So I would consider them safe in these terms.
Another point that might be worth a thought is their performance. Image the guy in front of a group is the only one to have disc brakes and does an emergency stop. With their bigger brake performance they might decelerate much faster than the ones with the conventional brakes behind them, causing all others to hit him from behind.
Disc brakes are reasonably tucked away on the frame and fork and as such pose relatively little danger relative to other components such as the large chain ring (which I have experienced in a peloton crash... Yay, stitches!). If you do experience complaints it would likely be either do to out of date rules or prejudices.
In terms of braking performance gap, I don't see this being a contributor to crashes as you should really never be using full brake power within a peloton setting. Braking should always be done with mindfulness of others and in a controlled fashion. If anything disc brakes provide finer control and could potentially be safer. In the situation of absolute emergency stops, yes you could out brake a rim brake in wet conditions which could cause a problem. However, if you are in the situation of an all out emergence stop within a peloton, something has already gone horribly wrong and really all bets are off in terms of outcomes. Under dry conditions I personally haven't notice any real world differences in total braking performance between road disc brakes and well set up rim brakes. Modulation, yes, but total power no. Maybe hydraulic discs would have a bit more power (I have only used cable disc brakes) but at some point you are hitting the traction limit of the tires and well maintained rim brakes should be able to do that in dry conditions.
As an aside, aero bars are considered unsafe due to the lack of fine control. Many riders drift in the aero position and tend to have slower response times. I have seen so many calamities over the years with people misapplying aero bars to regular riding (e.g., slow train track crossing!) I think most would be best to leave them at home unless on a dedicated TT course.