Does Magneton Deserve a Spot?
It has a niche for trapping Steels.
Magneton can potentially trapping and KO-ing your opponent Annoying Steel Types that don't carry a Fighting, fire or ground move, like Magnezone in OU
Potentailly taking out Foes like Empoleon, Escavelier, Durant, and even Registeel if Magneton carries a Air balloon or magnet rise.
It even Speed ties with Bisharp if Bisharp happens to max Speed EV. It'd come down to a coin flip to who hits first with brick break or HP-fire and Magneton Resist Suckerpunch, if Bisharp decides to go for reliable damage, if they know they can not out speed Magneton.
Magneton's only competition in UU trapping is Wobbuffet and Dugtrio.
Wobbuffet is a different Beast all together
While Dugtrio is very frail can can't really switch into anything that isn't a electric attack, and used more like a revenger.
The only thing Dugtrio is better at doing or just as good as doing in terms of taking out steels are Cobalion and Empoleon (although Empoleon has access to Aqua Jet but it's rarely used) Dugtrio won't be KO-ing Escavelier, Durant, Registeel unless significantly damage or Bisharp who has priority sucker punch.
Dugtrio obviously traps everything else better that doesn't Fly, Levitate, carry a Air Balloon or has access to Baton-Pass and U-turn
What about Bouffalant?
Although Sawsbuck does a similar job with Sap Sipper and has Greater Speed, Bouffalant has a Higher Attack and bulk and doesn't suffer 5 extra weaknesses from a secondary grass typing.
And Bouffalant has moves Sawsbuck could only dream of like Payback and Pursuit.
And with the Removal of Drought, Sawsbuck is likely to fall in usage as previously Sawsbuck was most likely used to leverage Chlorophyll.
Bouffalant can come in and wall, while gaining an attack boost from most of the Grass pokemon threats like Celebi, Shaymin, Roserade, Tangrowth, Abomasnow or anyone carrying a grass move like Venemoth's sleep powder.
Pursuit is especially interesting as it put Celebi in a checkmate position, especially if bouffalant is carrying an choice band and received a attack boost from celebi's grass moves (like leech seed or leaf storm)
It can also pursuit the many frail sweeping ghost type like Mismagius if they don't have Will-o-wisp.
and even if you don't pursuit, with a choice band and and a attack boost from Sap Sipper Combined with it's signature Stab Move Head Charge, it will do a lot of damage.
Other notable mentions I can think of...
Amoonguss
It's has access to Spore, The only other pokemon that have it in UU are Smeargle and Parasect, not much competition there, it also has Clear smog to Counter pokemon that may try to set up on it. I'm not very familiar with this pokemon so maybe someone else could expalin why it deserve a UU analysis.
Walrein
Deserves an UU analysis just to exploit Hail with Ice Body and reasonable good bulk.
Weezing
Is a maybe for me...I'm kinda split whether or not it should deserve a UU Analysis
Compared to other Physical walls, the 1st that come to mind are Tangrowth and Registeel.
Weezing is used entirely differently with it's move pool that has Pain split, will o wisp and Clear-smog/Haze and only 1 weakness being Psychic which can be supported with a dark type pokemon like Bisharp.
But then the likes of Dusclops/Dusknoir comes to mind with better bulk and similar move pool.
But Weezing has the advantage of better typing and ability and leftovers, can't be toxic stalled if you're comparing it with Dusclops... hmmm...
Sharpedo
I'm surprised it's not on the list, anything full evolved pokemon with speed boost, proabably deserve a UU analysis.
Cradily
Works well together in the sandstorm with Hippotas support, not to mention it Storm drain gives it a Water Immunity and raising it's Special Attack.