Within the past month I started the series.
Railgun S1 = fun in academy city
Railgun S2 = fun then macabre "You've been cloned 20,000 times, watch them die to a sadistic monster as we try to push him to godhood". Then Touma saved the day, and is never seen again for the rest of the season, even though he's a love interest.
Index S1 = In same universe as Railgun, but every few episodes is a new arc, always adding new characters. The season doesn't seem to build up to a huge climax, season finale just feels like another arc.
Index S2 = even more mini arcs, this time traveling all around the world, adding more characters, and this kind of cheapens each arc because they just become forgotten as soon as the next arc occurs. Season finale feels like a proper climax, city gets WRECKED.
Index S3 = all over the place as the characters travel around the globe, quality and writing is much less sharp. Even though you finally see the heads of English & Roman churches get overpowered, the season has less impact.
A big turn off is that you get introduced to all these characters, but later on don't see where they go because they get ignored in the next arc, and the arc after that as well. It's one of the reasons why the school festival arc was fun, you finally see all the return of these beloved characters you used to see all the time.
Another problem is that sometimes there are characters that are built up to be incredibly strong, but turn around and they get destroyed in fights. That Kanzaki is one of only 20 saints in the world that are supposed to be insanely strong, but every time she shows up in S2 & S3 she loses in just a few hits. Accelerator was godlike in combat, but in S2 he gets destroyed against somebody's who's pretty normal. Worst of all is Index who's got the most insane eldrich knowledge of any person in the world...but doesn't use any of it with the exception of 3 times in as many seasons.
First seasons were the sharpest. Series got progressively dumbed down until Index Season 3, despite bigger players introduced, the series began to also rely on gags that also cheapened the series. Didn't start out as a harem anime, maybe love triangle, but as time went on it was like every female character was in love with Touma. Index would bite Touma as a gag joke if she hadn't eaten in 5 minutes.