why is it difficult to design glass products using fused glass?

Answer 1

It isn't. Glass can be molded into any workable shape and fused to any known material with little difficulty. There is even an entire section of glass science devoted to the use of sealing glasses for making rigid connections between glass and other materials.

If you are referring specifically to fused glass as "fused silica" then the difficculty is in melting the silica. Fused silica is a term reserved for truely SiO2 glass. Melting silica requires temperatures in excess of 2000 C, which is well beyond the means of commercial melters. There are short cuts to making this glass such as leaching of a heat treated spinodally phase seperated sodium borosilicate glass and sintering the remaining porous silica body.

Answer 2

Both previous answers have elements of truth and flaws.
Fused glass - that is melting together broken pieces of glass or of glass powder or frit - tends (but is not required) to be weaker than poured cast glass (a liquid) because the fused glass tends to have thousands of tiny bubbles which are weak points leading to breakage.
While there is a whole field of technology involving glass being fused to various materials, the reason it exists is that most glass will NOT fuse easily to most materials and special formulas are required to match glass to metal, to semiconductors and even to other glass. I blow glass and do things with metal and clay with glass and the varying rates of expansion play havoc with casual use.

Answer 3

One (perhaps the main one)problem is that different glasses can have different coefficients of thermal expansion, so that as they cool from the fusing temperature, large stresses can build up between parts of the finished object, and as cool glass is brittle this can cause breakage. The solution is to use glasses with similar expansion, matched glasses are available for art and hobbyist glass fusing.