There is an absorption of energy in an endothermic reacn.
We cannot say that the products formed are unstable or not , the factor which decides whether the reacn. really proceeds or is an hypothetical one is the FREE ENERGY CHANGE of the system i.e.
if

G is -ve then reacn. proceeds whatever be the type of reacn.

H is the enthalpy change.
S is the change in entropy. T is the temp. at which reacn. is taking place.