Of course transfusions are not a cure-all. They have the potential to be deadly even. However, when warranted, they are the best that we have to offer, and their benefits overwhelmingly outwiegh the risks.
There are three main reasons that a substitute for whole blood is being sought after. First, elimination of that small percentage of blood that carries harmful pathogens, second, the shortage of blood that the Red Cross is forever fighting against, and third, the limited shelf life of whole blood.
However, your suggestion that JW’s are being cautious because of the science is simply ludicrous. Transfusions have been accepted medical practice for many years, and the JW’s have resisted scientific evidence during that time because of their religious beliefs. They have however, latched on to any item that notes any of the dangers to try and make their Biblical interpretation seem reasonable in scientific terms, while simultaneously ignoring as pointedly as possible the evidence that supports transfusions.
If they wish to have a Biblically based ban on transfusions, that is an argument open only to theological counters. However, they keep trying to use science to bolster their religious sentiments, and it simply doesn’t do that job very well. It’s exactly like Creation “Scientists” fighting to make science support a literal interpretation of Genesis. The evidence doesn’t support the argument.