Regieleki is kinda in a weird spot. It's definitely broken, as in it's fucking insane when you have it launching modest specs + transistor + stab + Terrain boosted rising voltage. The problem is, it's only insane when clicking those electric moves, not necessarily making it unhealthy. Something unhealthy(imo) is a mon that forces certain teams to run a mon that they would normally not want to run, in this case, eleki is forcing all teams to run a pokemon immune or a bulky resist to electric. I'm only gonna go over the types that are affected by this, it really isnt that many.
Fire: Fire is now forced to run alolawak. wak itself isn't bad at all, and it's pretty easy to fit. However, alolawak is not a ground type, and therefore is affected by regieleki's one coverage move, ancient power, which ohkos if you get just a little bit chip on wak. This shouldnt be too much of a problem if you keep wak healthy, as running sr or spikes is kind of a waste of a slot in pinc or stunfisk. However, wak is really nice for deterring the eleki player to spam electric moves, forcing it to instead either act as a sack or click its much weaker ancient power.
Electric: Electric itself is kinda weird, as you'll be in an eleki v eleki mu, but most of your team resists it, and won't be ohkod, so eleki is not the real threat here.
Rock: Rhyperior is probably the one better rock/ground, but it's not like rock has to run rhy, stuff like cradily in sand can eat but the key thing is needing sand to actually take the hits. This isn't too big of a deal tho, stuff like Power Herb nihilego can start snowballing really well vs electric and scarf terrakion does outspeed modest eleki.
Fighting: Fighting has no pokemon at all immune to electric. However, fighting does have virizion, whos screens sets with synthesis can take rising, but the main thing here is that fighting's playstyle is very very offensive, so not having a good switchin isn't too bad, there are a lot of mons that fighting can not switch into like specs lele. Instead, fighting can offensively check eleki with scarf terrakion or any other scarfer that can ohko regieleki and hit 500 speed.
Psychic: Psychic is pretty interesting, Claydol is obviously unviable, but there is scarf latias, who is 3hkod by modest eleki, and also outspeeds and ohkos back with draco meteor.
252+ SpA Choice Specs Transistor Regieleki Rising Voltage (70 BP) vs. 0 HP / 4 SpD Latias in Electric Terrain: 121-143 (40.1 - 47.5%) -- guaranteed 3HKO
252 SpA Latias Draco Meteor vs. 0 HP / 0 SpD Regieleki: 328-387 (108.9 - 128.5%) -- guaranteed OHKO
The one thing here is, like maroon pointed out, thunderbolt will deal more damage, as latias levitates, and thunderbolt does 2hko latias, meaning latias can not repeatedly switch in
252+ SpA Choice Specs Transistor Regieleki Thunderbolt vs. 0 HP / 4 SpD Latias in Electric Terrain: 156-184 (51.8 - 61.1%) -- guaranteed 2HKO
I still do not believe this makes it broken, psychic has terrain of its own, and latias can very easily run a bulky set with recover as well.
Ghost: Ghost now has to either run wak(who's one problem is listed above) or a ground type, in palossand/runerigus/golurk. Palossand is pretty cool, it gets recovery and ep hits pretty hard vs electric. Golurk can run a nice cb set to pressure a lot of stuff with its iron fist punches and Poltergeist. Runerigus is the bulkiest and the lightest(takes less gknot damage) who can set up tspikes to boost its teammate's hexes from mons like pult or spectrier. Overall these mons are not exactly ghost's top choices, but they are not terrible, and add there own traits to ghost.
Dark: I have not used or seen anyone use dark that much, so i don't know for sure, but probably you'll need to either run av ttar or krookodile on your team to avoid regieleki sweeping you. Other pokemon like Hydreigon or Zarude can probably take a hit and ko back.
Fairy: Fairy also has no ground type or immunity, but Tapu Bulu can set its own terrain and with klefki's screens, it can take rising voltage incredibly well
252+ SpA Choice Specs Transistor Regieleki Rising Voltage (70 BP) vs. 0 HP / 0 SpD Tapu Bulu: 123-145 (43.7 - 51.6%) -- guaranteed 3HKO after Grassy Terrain recovery
he can not directly switch in without screens, so he isnt entirely foolproof, but as shown here
https://replay.pokemonshowdown.com/gen8monotype-1209914823
horn leech+leftys+terrain means that bulu can constantly keep himself healthy vs eleki, and eleki in that match was not able to come in that much at all.
anyway that's all, eleki seemed to be a hot topic and i 100% agree with the ppl that posted before me, eleki is NOT oppressive on teambuilding or enough types that it is unhealthy. Most of the types i listed are doing fine even when being forced to use Pokemon not considered viable before, and eleki's damage output when the opponent has no checks is not enough to make it ban worthy in my eyes.