Filters used in FBMC
long decaying tails which can make them robust

Why would filter long decaying tails make a signal robust

can make them robust, not makes them robust!
You need a long filter length for narrow bandwidth and steep transition width.
So, with a long filter, you can make very narrow subcarrier channels.
Those being narrow, they don't &quot;see&quot; multipath, they only see a flat channel.