The visual effects (CGI) and the third act were heavily criticized for feeling "unfinished" or "tacky". Box Office:

Worlds collide as Barry faces the consequences of his actions, ultimately learning that some events in time are "fixed points" that cannot be changed without catastrophic results. Reception and Performance

Barry attempts to save his mother but ends up in an alternate reality where General Zod has returned, and there are no metahumans to stop him.

This new timeline is chaotic: