Depends on your bar. Plotwise, it's just "cosmic bad guy wants to destroy everything for as-yet unexplained reasons", which is easy enough to roll with. Everything has only been vaguely hinted at till now, so you'll learn all the why's alongside the people who've been following the shows.
Character-wise, you're gonna be lost, but that mostly won't be a big deal. If you're just checking in for the crossover, you probably won't be that concerned about each hero's 5 non-costumed friends and their interpersonal relationships.
The only thing that might get you is the alternate Earths reusing actors (like John Wesley Shipp is 90s Barry Allen on one Earth, Jay Garrick on another, and CW Barry's dead father), but even that will probably be covered in dialogue.