Yes, they have their issues and yes, many consider them outdated but, personally I am a big fan of real print and paper books. Call me old-fashioned, I like holding them in my hand and being able to flip through the pages. That being said I don't think I would like the Mini-guide due to it's size. While the size would make it convenient to fit in a pocket or backpack, I think it would just be too small for practical use. While the written information is important, the pictures are the biggest help to me initially and I think they would just be too small. I like to be able to easily differentiate between male, female, and juvenile and it doesn't seem that such a small picture would be able to readily show such detail. Also while there are many young birders, It seems to me the majority are middle-aged or older. I myself started bird-watching at a very young age at the side of my older than average and poor sighted mother. She was always handing me the field guides we had because my vision was better and she couldn't make out the details good enough. I always had excellent vision until recent years, and while I do wear glasses now I still struggle with the fine print on cans and such. I guess the point I am getting at is that considering a goodly portion of Birders are retirees and older in years, many might find the small print of the mini book highly annoying. I don't usually use apps while in the field but I do snap multiple pictures with my phone quite often. Then when I get home I will review the best of them and search in my books and online for appropriate matches. I prefer more detailed information to quick summaries though. I want to know range, habitat, food preferences... I used to buy every book I could find on my subjects of interest. Now I find I am more discriminating and don't invest in a book unless it offers new and interesting information or is incredibly knowledgeable.
Best wishes with your book, no matter what form it takes. Shared knowledge is always a good thing..