Tobi even said that the Senju was the strongest clan. Everytime someone hired Senju the opposing group hired Uchiha because Uchiha was the only group who could do battle with the Senju. If we go based on the facts the Senju are the most powerful. Uzumaki was so feared that everyone wanted them gone due to there sealing jutsu and massive chakra. Uchiha is looked down on and not every Uchiha had the sharingan and only a select few obtained it. Thats why Tobi said getting the sharingan made them special in the clan and obtaining the mangekyou allowed him to take control over the clan. Each clans best faught(Senju and Uchiha) with the Senju winning. Madara had EMS at that time and still lost just like his ancestor lost to the Senju's forefather. Is it also any surprise no bloodline trait to use special elements also doesn't have a doujutsu? Only Shodaime used wood and thats a known fact that only he could use it with Yamato being a copy of him. the Sharingan fails against special elements like ice, lava, acid, dust, storm etc...
"Uchia was looked down on"? you implied that senju was the most powerfull clan in history, then went on to say the only clan who could stand up to them was uchia, so how were they looked down on?. The truth is uchia and senju were evenly matched non of them could completely wipe out each other (that's a fact). Tired of the blood shed hashirama, offered them a peace treaty which they graciously accepted.
In my view the only reason why senju was regarded as the strongest between the two, was because one represented evil, hatred, revenge, war, power (envied senju because the sage chose him to be his successor) etc, while senju represented good, using love to cure war rather than strenght and might, will of the fire (to protect their comrades and strive to bring peace to the world).
Sharingan doesn't fail against special elements, it just fails to copy it (big difference). A sharingan user can own a user with a special element any day.
In the history books senju is the strongest clan, in my opinon uchia is.