What the 3m rep said was true, but the fact is it may be ph neutral now, but these things do change. It is not an "archival" product by any stretch of the imagination. I am a professional picture framer and have been for 20 years. I have studied conservation framing and archival permanence issues. I would equate the ph neutral quality of 3m adhesives (77, Photo Mount, Vacu Mount, etc.) to that of a "paper" or "regular" mat board. The companies that manufacture these call them "acid free"...but in the case of these products, they have a buffer that makes them ph neutral and this is what they are referring to. Only problem is that this buffer breaks down over time, the mat darkens to a shade of yellow or brown and starts leaching acids into whatever it's touching.
In the case of the spray glue adhesive, and even dry mounting methods, if you want or need your item flat, then go ahead and do it. It's not an "archival" way to do mounting because it is not reversible. There are dry mounting tissues (like one called "Fusion") that claim to be reversible, but I've never tried it. I tell my customers that it's a trade off... and ask them "Do you want your art to be absolutely flat, or do you want it to be preserved for the next 20+ years?" If your goal is to sell to the general public or for presentations where the image is mounted directly on top of a mat board, then obviously, you have to go this route, but if you are making nice fiber paper prints mounted in window mats, then you can use better archival methods.
Sorry if I'm rambling! Obviously conservation framing and photography are things I'm very passionate about ;-)