While Heliolisk and Magneton are not directly comparable, it is easy to draw parallels to each other: both gave the tier trouble due to the power of their Volt Switches, and how difficult they are to block. The powerful Volt Switch wore down switch-ins much faster than they can attempt to heal off the damage (Ferroseed being a great example here), while their access to powerful attacks to smack most Volt Switch absorbers meant that players are restricted in their available responses. The fact that feasible counters to Magneton (Lanturn, Stunfisk, Torterra, Quagsire) exist meant that Magneton is less broken than Heliolisk, but it has its own problematic qualities, mainly its sheer power and good defensive typing which grants it a good amount of switch-in opportunities, plus its stat distribution give it the ability to threaten any playstyle with the respective item (Scarf for offense, Specs for balance / stall, Eviolite is arguably an all-rounder tho it fares its best against balance), making it a significant universal threat all teams should prepare for, and when you account for the limited counters to Magneton in the tier, well.... (also Magnet Rise can fuk with some people, most notably beating Stunfisk 1v1)
Even if Magneton isn't outright broken, there is no denying it restricts the diversity of the metagame. Most teams need a Magneton response to avoid getting punched open by one of its various sets, and its main hard counters are all susceptible to Grass-types. Of course, pairing a Grass-type alongside Magneton opens up the team to Fire-types, which means responses to Fire-types need to be run, and don't forget you need to be prepared for Magneton as well... Already this seems pretty formulaic, even the likes of Archeops didn't manage to limit diversity to this extent (you can pair Ground-types or Fighting-types alongside Archeops). Of course, the alternative to defending against Magneton would be to pack a Ground-type + a mon that resists Magneton's remaining attacks (which isn't even foolproof since Magneton is not guaranteed to be Choiced), but when the solution against a threat in the tier is to either check it with two mons or pack one out of a limited number of reliable responses which all happen to be susceptible to a singular type, this makes teambuilding for the defending player rather tricky, and on the other hand remarkably streamlines teambuilding for the Magneton user. This is not a good indication of meta stability / diversity and is a prime example of overcentralisation, and Magneton needs to leave the tier as soon as possible for the benefit of the meta.