Could someone help me explain what's so particularly bad about Iku, Yuugenmagan, and Technical Reisen?
No one's really explored some of the stuff in D Rank so they go almost entirely unused. Players are quick to reach for more consistent puppets.
The Iku's aren't bad; they're just extremely awkward and only seem to work well on teams that appreciate Paralysis support. Iku's can be annoying if played right with a little luck on your side, though. Iku and Defense Iku can run Static, so when they switch in to a move that requires physical-contact, the opponent has a 30% chance of being Paralyzed. Iku can then take advantage of her incredible base 130 SpAtk stat to hit the opponent with her powerful STAB Thunderbolt. Defense Iku can take advantage of a Paralyzed opponent with Knock Off. Technical Iku is different in that she actually has a somewhat workable Speed stat that she can abuse with Charge Beam or even Icy Wind to punish would-be checks and counters.
The big reason why I think Iku isn't used is because she's extremely predictable. Iku's most powerful attack is Thunderbolt, and her only way of dealing with Earth and Nature types is with Water Pulse and Razor Wind respectively coming off her incredible SpAtk stat. Defense Iku creates very little offensive pressure (though her Thunderbolt still hurts), and Technical Iku is just way too gimmicky and is outclassed by other Wind-types. The best thing about Iku is that she only has one weakness. Not a bad puppet, but very difficult to slap on a team, even one that needs Paralysis support.
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Yuugenmagan gets unfairly compared to Defense Hina. Yuugenmagan can't spin, and it's not immune to Earth-type moves, but it takes Physical hits a bit better, takes Special hits WAY better, is slightly faster and slightly stronger specially, heals more with Leftovers, and has an undervalued stall-oriented ability in Pressure. Unfortunately, Yuugenmagan's move-pool is shallow, it doesn't learn Will-O-Wisp, and the rest of the tools that Defense Hina has available to her are more valued in competitive play.
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Tech Reisen trades 15 points of Speed and SpAtk for 25 points of Defense and 5 points of SpDef with her original evolution. Tech Reisen also trades Mana Charge, Sonicboom, and Heart's Eye for... Encore, which she barely has the speed to run now. Even if you do need Tech Reisen's Defense for a supportive set, there are better defensive Heart-types, and better Sleep-sponges, so unless you absolutely need both of those things, and know how to use a Damage Calculator to make EV spreads, then there will probably be a better option.