I'd say sun is also a bit of an issue, though less so since the sleep ban, but I think the clear problem is the 7 turns and how complimentary both barra and arch are, they cover each other's weaknesses so well. Arch's main weaknesses is it's speed tier and difficulty with ground types, as although it can destroy tusk, it can't take a hit from it if it hasn't got a stamina boost while being at high hp, which probably won't occur since the opponent will be wary to use u-turn and other low power moves if you have arch on your team. Barra's main weaknesses is it's fraility and difficulty with some physical walls, though those are few and far between. Together, they can solve each other's issues. Combine them with a special wall, and you have perfect synergy. Then add on the fact that you have to deal with this for seven turns at a time, and then it is unstoppable.
The difference between this and excadrill is that sand was permanent, and they eventually banned sand rush, so we have some precedent on not banning the weather.
Also, I was looking at mons that get rain dish, and couldn't specially defensive ludicolo pair pretty well with these two mons, as it resists ground moves while being neutral to electric. It can get up a leech seed and support the team further.