There were parts of it that were well done. Having the two Barry's, one older, one younger, was absolutely believable. Later adding Dark Flash also worked. Michael Keaton Batman was fantastic. As you say, Supergirl was excellent.

But the only way to enjoy it is to look past the really awful VFX shots. I give them total props for putting in George Reeves, Adam West, Christopher Reeve, Helen Slater, and freakin' Nicholas Cage fighting a giant spider... but man... the shots were plastic. Some kid could have done a better job with action figures.

Even if you want to argue that the Time Bubble/Cross Universe stuff is supposed to look distorted, that still doesn't excuse the Babies in Danger^TM sequence.