1. Too many speedsters. Flash, Reverse Flash, Zoom, Kid Flash, Excess(Flash's daughter from the future), Jay Garrick, Jesse Quick, Iris West(For 1 episode), and That speedster from Earth-19 who is shown for 5 seconds in a couple of season 3 and 4 episodes. And 90's Barry Allen (appeared in Elseworlds on Earth-90)
2. Too many Harrison Wells'. Eobard Thawne as fake Harrison Wells for season 1, Harrison Wells from Earth-2 for season 2, Harrison Wells from Earth-19 for season 3, Council of Wells each Harrison from different Earths. The funny thing is we never get to see the real Harrison Wells execpt for the flashback in season 1 when Eobard Thawne kills him and turns his body into Harrison Wells.
3. Reverse Flash never dies. He appears to be erased from existence in season 1, but he comes back in season 2 for 1 episode, and gets sent back to his own time. He then comes back for 1 episode again in season 3 of The Flash, in the Flashpoint episode, then goes to Legends of Tomorrow. There he unites the Legion of Doom in season 2 of Legends of Tomorrow, and then gets erased from existence again! He comes back in Season 4 of Flash for Crisis on Earth X which is revealed that he is still alive. He most recently came back in the episode in season 5 of The flash where The Flash goes back in time to Savitar, Zoom, and Reverse Flash. Later in the episode it goes to the year 2049 or something and it is revealed that Reverse Flash is in jail working with Nora.
3. There is a lot of CGI when they are too lazy to actually shot shots. And sometimes they there are too lazy to even cast a villan character so they use CGI as the villan like Savitar in Season 3 who was completely CGI and when he was not in his suit, he was revealed to be Barry Allen which is the most Cliche thing they could think of.
4. The only good part is the annual crossover which is REALLY long (as in 4 whole 52 minute episodes.)
5. It's a CW show