Why is securitization risky?

Why is securitization, the process of pooling various assets and turning them into tradable securities, considered risky? Could it be due to the complexity of the underlying assets, which may be difficult to value accurately? Or is it the potential for default or delinquency within the pool, which could lead to significant losses for investors? Additionally, could the lack of transparency in the securitization process, particularly in the case of asset-backed securities, contribute to the risk? Ultimately, what steps can be taken to mitigate these risks and ensure the stability of the securitization market?