I've been thinking about this for a while in my USUM playthrough, everytime I use hammer arm i'm left on a weird position, not being able to outspeed anything when I could before, sometimes costing me a faint against dark types. What should I do?
Definitely Hammer arm.
It's really powerful sometimes KOes things which brick break can't. For your speed problem, many dark already outspeed Metagross. But anyway, after finishing off a Pokémon with hammer arm, simply switch out!
Hope I helped!