Clef looks threatening. From experience, having Exca as your main check doesn't end well because it's always eventually worn down, especially when it's the primary switch in to Moonblast. Every other member kind of loses 1v1, and the other main ways of applying offensive pressure on Clef besides Exca (Mega Zam and Torn-T) are crippled by Thunder Wave. Tangrowth can hope for a Sleep Powder, but realistically, I don't think anyone would risk letting their Clefable fall asleep versus this team given the work it's able to do. In the long run I think Clef will definitely give this team a hard time.
Aside from Clef, the other problem I see are offensive breakers capable of exploiting the team's defensive core of Tar, Rotom-W, and Tangrowth. For example, Keldeo, Mega Zam, Mega Diancie, Mega Cham, Mega Hera, and Mega Gardevoir all apply a great deal of offensive pressure on this team once they find the chance to come in against one of the team's defensive members. The team has ways of checking them offensively between Exca, the team's own Mega Zam, and Torn-T. However, they're still scary for the team simply because they threaten to kill or heavily damage something each time they're in. This can be problematic versus a player who realizes this and plays aggressively enough to take advantage of this weakness.
Although it's not too common anymore, Zard-Y also falls under the category of Pokemon this team has a hard time switching into. Like the offensive breakers mentioned earlier, it's able to threaten the team's defensive core well. However, the main thing separating Zard-Y from them is its ability to remove sand and threaten to kill Exca before being killed itself. Another factor is Zard-Y doesn't fear Torn-T or Mega Zam as much as other problematic threats like Mega Cham, Keld, and Mega Hera. In practice, Zard-Y's success against this team will probably depend on how well SR can be kept off, but it's still an issue to consider in any case. One more Pokemon I can see being annoying is Mega Scizor, primarily if Rotom-W dies or gets worn down too much (which isn't unrealistic if you consider sand, SR, and the lack of Leftovers/bulk). A classic combo like Bisharp + Mega Scizor, for example, can threaten to sweep the team handily.
Defensively, I can see something like Mega Eye being a big pain. It's able to stop Tar from setting up SR, and nothing on the team really likes eating a WoW or Knock, which can both serve to quickly wear things down here. When found on stall and paired with your typical defensive backbone (Weavile stall as an example), it's definitely going to be a nuisance. This is tied to the lack of solid tools this team has versus stall in general.
Overall, I think most of the weaknesses here stem from several offensive Pokemon being capable of exploiting the team's defensive core, as well as a large reliance on Exca to handle fairies and several faster threats. The team has to be played in such a way as to minimize the opportunities these offensive threats have to threaten the team. This is done by applying your own offensive pressure with Mega Zam, Exca, and Torn-T before the person you're facing can pressure you themselves. It's a cool sand team all in all and has a good mixture of defensive and offensive options.