I do not think it has much of an affect at all. It is a very low ph, maybe even check it if you have the means. Soak some peat in water for a week or more, then test the ph. I believe it doesn't get below like 6, that might even be to large of error on my side. Some tadpoles, like those of some mantella's, like the water to be slightly acidic. So, unless it is a really low ph, I do not believe it to be harmful. I am no pro though, I am sure someone who really knows for sure will chime in.
I think you'll find a lot of people on here that use it. Its easy to get and easy to use. Its all on personal preference and desire for maintenance. We use cocofiber and dirt for the most part. It's a little messier but you can always seed the substrate. We have tried using sphagnum as well but that caused some acidic environments for us. All in all peat seems to be safe to use. I don't think there are any academic publications but if you're worried just test the pH before you use it.
Thanks for all the replies guys, I was mostly concerned abut because of the same stated above with the sphanum moss and acidic situations. I was also concer with the use of all peatmoss, but ia m gonna mix it with a vivarium mix as well as some orchid bark for better moisture retention.
The tannic acids in peat are beneficial. It acts as an antifungal agent for both the soil and tadpole water and it helps drop the pH of soil into the slightly acidic range where most plants find it easiest for nutrient uptake. In water it also creates slightly acidic conditions which are favorable. I think the pH of 6 mentioned is about right when it is diluted in either a terrestrial or aquatic environment. The acidity of pure peat is not near low enough to cause chemical burns. It's absolutely safe and beneficial.
BTW, this is the same acid you get with oak, avacodo, almond, and a wide variety of leaves commonly used to produce tadpole tea.
On a side note, peat and sphagnum are not safe for most caudates and kill or severely stress the caudates through ionic imbalances. That said, I use both (both historically and currently) for many different anuran species with no problems.