Under no circumstances should they be substituted for each other! This is a matter of principle, and incorrect substitution can lead to serious consequences.

In simple terms, their fundamental difference lies in their core functions:
Structural adhesive: Its core function is to bear loads (structural strength).
Weather-resistant sealant: Its core function is to resist climate changes (seal durability).

1. Why can't weather-resistant sealant be used as a substitute for structural adhesive?
This is an extremely dangerous behavior. The strength of weather-resistant sealant is very low, and it cannot withstand the load transmitted by structural components at all.
Consequences: If weather-resistant sealant is used to adhere a piece of curtain wall glass or stone, over time, due to the effects of wind load, self-weight, and thermal expansion and contraction, the adhesion will fail, leading to loosening or even detachment of the components, which can result in serious safety accidents.

2. Why can't structural adhesive be used as a substitute for weather-resistant sealant?
Although it appears to be "stronger", there will still be issues with this approach:

Consequence 1: Insufficient elasticity, leading to cracking. Many structural adhesives, especially high modulus products, have high hardness and poor elasticity. When used to seal expansion joints, they cannot adapt to the thermal expansion and contraction of the substrate, and will eventually be pulled apart, resulting in sealing failure, water leakage, and air leakage.

Consequence 2: Insufficient specialization in weather resistance. Although structural adhesives are also weather-resistant, specialized weather-resistant adhesives have undergone extreme optimization in their formulations for UV resistance, anti-aging, and other properties, and their long-term weather resistance is generally superior to that of structural adhesives. In purely outdoor exposure environments, the lifespan of structural adhesives used for sealing may not be as long as that of professional weather-resistant adhesives.