Since when was Tobirama afraid of the Uchiha?
Come on. As stated in the manga, the only Uchiha he was shown to be frightened of is EMS Madara:
And EMS Madara is right under Hashirama, who was stated to have made the five current Kage's combined efforts look like nothing, and be above the likes of DSM Kabuto. Who wouldn't be wary of another EMS Madara appearing? Seriously.
Even after Sasuke displays his Eternal Mangekyou Sharingan, Tobirama still refuses to respect him, that must say something in and of itself (although Tobirama believes it only to be MS):
The assumption I can make from this is that your-average-MS-user isn't up to par with Tobirama (once again, because Tobirama isn't aware that Sasuke actually has EMS).