I found it difficult to work with as it was really soft. It pushed out through the grout joints, got every where and was a real pain to keep the area clean. Sponges, although cheap need to be disposed of after using the stuff for instance. trivial I know but a cost and hassle all the same. I was very inexperienced at the time though and have not used it since, preferring to use Ardex 7001.
As for the 18mm ply, I feel that if the floor moves much, it should be strengthened. Fastflex is great if you have a small area but I would be a bit uncomfortable using it over a large area. I may be wrong but you only seem to get about 3.5m2 for a set so anything reasonably large would be very expensive at £40 a set, and it goes off real quick in the bucket.