A decent epoxy will remain slightly flexible, better ones are slightly more flexible than ones that are glass hard.
`Glass hard` ones are rubbish... and shouldnt be used for most things they have little or no impact resistance and very low sheer strength
The slightly flexible ones are are known as toughened the highly flexible ones as flexible , standard model trade epoxys the cheapo types of all speeds (especialy the std cheap epoxys sold for wing skinning) are more brittle and have no impact resistance, and in adhesive formulations have low sheer strength, a toughened one is always better than a hard one, even though a hard one may look like it will be stronger
Adhesives from decent manufacturers like our StarLoc and some Loctite epoxys have very high sheer strength, high tensile strength and great impact resistance due to the toughened formulation
A lot of diy shop grades are just epoxy one one side and the hardener on the other, giving a harder set and far lower strength