Assuming the moves listed on Melmetal's page are accurate, it won't be any good as a steel-type attacker, since it doesn't learn any steel quick moves, and its only steel charged move is Flash Cannon (which is a pretty bad move).
As an electric-type attacker, its lower on DPS than quite a few other Mons, but has a ton of bulk. Its TDO should be decent.
As a defender, I imagine it will be decent given its high defense and stamina assuming that placing it in gyms is an intentional feature (we can't put other Mythical Pokemon in gyms). Though it ends up as another gym defender that's weak to fighting and doesn't offer meaningful counter-coverage against Machamp.
GamePress
Does melmetal have a place in the (mel)meta?
Is it useful as steel or electric type attacker, is useful as a defender?
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Answers
No steel quick move, and being stuck with Flash Cannon as a charge move means you may as well forget about using it as a Steel attacker. Really not worth it.
While it does have an Electric moveset, without stab, it's not likely to stand out much. It doesn't hold a light to the likes of Electrivire, Zapdos, Raikou, Magnezone, or even Luxray. I suppose it's a usable Electric attacker just due to it's hefty stats.
Luckily for Melmetal, Niantic let us put it in Gyms. It definitely has a great stat spread for gym defense. Although the Fighting weakness doesn't help when it comes to keeping the gyms from being Machamp food. Pure Steel is one of the best defensive typings in the game, offering loads of resistances. The only real issue with using Melmetal as a defender comes from it's quick move. Thunder Shock is pathetic on defense. Not that damage output is super important when defending, but it would be nice if Melmetal had another quick move option to apply more pressure to attackers. On the bright side, Rock Slide and Thunderbolt are decent charge moves for defense. Ground types will give it a bad day, though.
All in all, I think Niantic really gimped Melmetal by limiting it to a single fast move. Currently, in the Lets Go games, Melmetal only learns two Steel moves: Flash Cannon and Double-Iron Bash. The later sounds like it would also be a charge move, so unfortunately, I don't think Melmetal will ever be a good option for Steel offense.
That being said, it's still not a bad defender. Plus, it looks awesome. Put 'em in a gym and show 'em off!
For defender i dont know, i am not really care about gym defending since the gym system change.
For attacker, if usefull is meta relevant, i choose Metagross CD moveset at steel dept.
for electric attacker, i prefer using the electric type pokemon to do the job.
If you can comfortably find enough people to do any raid, i think it is good for a cool factor.
The attacks listed on the Gamepress page for Melmetal don't really mesh up with the attacks listed in some of the flavor text from Niantic. Supposedly it has a new move, double nut bash, where it spins it's body around and hits with both hex nut arms. Should be a lot better than Flash Cannon. We'll see when someone gets one, and it wouldn't surprise me for it to get a steel fast move too.
Melmetal doesn't currently learn any of the steel fast moves in Let's Go so it won't see them here. In fact, Flash Cannon and its signature move are the only two steel type attacks in Let's Go that it can learn and its signature hasn't been added to PoGo. Let's Go also gutted like 500-some attacks from the previous games. When they return in the 2019 game we'll see if Melmetal's movepool expands and if so if its movepool in PoGo gets rectified to match.
Until then, what you see is what you get.
