Hi, 3d addict.
Any scale design will suffer a bit in thermals compared to a purpose designed model. That's just the nature of the beast. Generally the biggest problem is that wings with a high aspect ratio tend to tip stall in flat, tight, slow turns rather than a lack of ability to climb in lift. I don't have an FMS ASW-28 but, as is obvious from the thread, it seems to soar quite well. I do, however, have a ST ASW-28.
It takes some work to get it flying well but it is a good flier and responds well to light lift. I wouldn't say that it's fragile. All I did to stiffen it was add three coats of paint and I have had no problems with flutter even when bailing out of monster thermals at high speed. I've also hit a tree and a light pole with no damage aside from a dent in the leading edge. The big problem is the stock ailerons and linkage which are nearly useless. I enjoy flying it but everything I've seen indicates that the FMS is a better performer and a better quality product straight from the box but neither will float like a Radian or similar glider. If you have questions about the ST Models ASW-28 feel free to ask me on this thread: