I do not think that many people are using a Kindle as a communication device. I am very sensitive to the fact that many teachers cannot get the money they need or want. When you can get the funds, the process is slow. It takes a lot of work to show that something will work or is worth wild. Kindle is very reasonably priced. If that is what you have, then it is a good option. In order to get the administration to do something, the shortest route is proof of it working. If you have not tested something out, then you have no idea if it will work. If you recommend a technology teach everyone how to use it and go through the entire purchase order process to find that it is not being used that is a lesson in frustration. Having less expensive options can open needed doors in the future.
This might be a consideration because the price of a Kindle Fire is reasonable. Amazon also takes purchase orders. If you are trying something out on a fixed budget, this might be worth a look.
TalkTablet - speech/autism/aphasia/AAC Honorable Mention
This is an app that was already in the book on a different platform. This is a good app.
Speech Assistant AAC
This is a free app that you can try if you have a Kindle Fire.
LetMeTalk: Free AAC Talker
This is a free AAC app that gets good ratings.