Contested Sequel: To players introduced to this series in its beginnings, pretty much any game after Raiden DX garners criticism from that portion of the fanbase:
Raiden III is forever remembered as "that game that ditched and replaced the toothpaste laser". Players also note how soon you enter outer space compared to the previous games (stage 4 instead of stage 6 in the previous games) and the lackluster stage designs and enemy attack patterns. Released two years later, Raiden IV addresses these complaints and added back the Bend Plasma, along with revamping the Proton Laser, however...
Raiden IV is criticized for two sets of very short main loop of five stages (seven in the home console modes), secrets (Sol towers and Micluses) that you have to go far out of your way to obtain, the charged missile shot mechanic, and not much difference between the three selectable fighters (two of which are DLC in the original version). Some of these complaints were addressed when the Overkill rerelease came out in 2014.
Raiden V is criticized for changing many long-standing game mechanics and having a story that is considered by older players to be unnecessary. It also receives a lot of criticism for not including a vertical orientation optionnote which the previous versions needed since they were ported from the original arcade versions, which use a vertically-oriented monitor setup. Raiden V never had an arcade release, being released straight to the Xbox One, so therefore, it was designed with the console approach in mind., along with the addition of social elements and a soundtrack not composed by long-time series composer Go Sato.
Porting Disaster: The SNES version of Raiden Trad has terrible graphics, ton of slowdowns and arbitrary changes to the gameplay.
Replacement Scrappy: The Proton Laser in Raiden III, which serves as a replacement for the Bend Plasma and was panned as a result. If you ask anyone who doesn't like III why they don't like it, the answer will be "it doesn't have the Toothpaste Laser" more often than not.
Rescued from the Scrappy Heap: The Spread Shot default weapon is heavily disliked in the sprite-based games due to requiring rapid tapping to get its damage-per-second output to a respectable level and plenty of powerups to give it a good spread. From Raiden III onwards, it starts off as a three-way spread and can be rapid-fired simply by holding the fire button down (while the other two weapons have this behavior, the spread gets the highest quality-of-life improvement out of it).
In Viper Phase 1, you get a multiplier applied to your end-of-stage bonuses dependent on what percentage of enemies you killed. If you destroy every single enemy, the muliplier is x100. But if you so much as miss a single enemy, that multiplier drops to a x50. It won't matter much in a survival-oriented run, but in a score-based run, missing one enemy can make a massive difference.
Raiden IV's Light Mode is a Scrappy Mode at its best, for those who are lazy or don't bother to go on game settings. Some knowledged players would rather play Original Mode (or in home ports, Additional Mode or Overkill mode) in Practicedifficulty over this. The inclusion of Overkill mode in Overkill ports render Light Mode obsolete.
"Seinfeld" Is Unfunny: Throughout the 90's, Raiden, II, and DX were seen as the face of the Shoot 'em Up genre, and was well-known for its purple "toothpaste laser". The proliferation of Bullet Hell games in the 2000's and 2010's have not done wonders for the series' popularity; nowadays it's seen as having lifeless, generic visuals and lacking pretty bullet patterns, not helped by the additional difficulty that stems from the huge hitbox and loss of most powerups upon death. Although new games in the series continue to be released today, Raiden IV and V in particular ended up going under the radar.
Serial Numbers Filed Off: One can consider Raiden to be a Toaplan-styled shmup. It is in many ways a retread the Toaplan shmup Twin Cobra. The homing missiles also seem to have taken their trajectory routines from Toaplan's Slap Fight/ALCON. On top of that, most of the music homages music from other Toaplan shmups (see Suspiciously Similar Song for specific examples).
They Changed It, Now It Sucks!: The Proton Laser in III. Instead of being purple and twirling all over the damn place, it's green and only swings from left to right depending on the player's movement, its only real advantage is that it pierces through enemies while damaging them. The outcry was such that in Raiden IV, you get to choose between the Bend Plasma and a triple-beam (and purple) version of the Proton Laser. The way the Bend Plasma behaves in IV and V is different to II, DX, and Raiden Fighters. The suspicion among some fans is that Seibu had simply lost the original algorithms, and only created the Proton Laser the way they did because of that.