Already encountered it.
Senna/Yuumi is toxic. However, it's specifically toxic vs other Sennas (because one has 2 shields and 2 heals and stat Amp, and the other doesn't), so I do have limited Experience outside that exchange.
However, if snowballing, by the stacks where Senna would normally be around 400 AD, the Senna/Yuumi combo has around 550 AD and begins critting near 980. It's toxic with her range increase.
However, I DID see windows where if I was playing other supports, I could have drastically prevented her from snowballing, because early game, Senna is like half an adc. If I could have punished her early, she probably would have fed 5-6 kills, and gave up some 30-40 CS and 20+ stacks of Mist. Unfortunately, I was supporting a Yasuo adc at the time (This is what Senna's real purpose is; to get Yasuo out of bot lane)
Bottom line: If you are in a lane that isn't semi-trolling/playing an off meta, you'll be fine if you play aggressive and shut her down. If you are in a lane with a Yasuo adc, or something like it, Expect to lose, because Senna's autos count as Lasers and bypass Wind walls, but hold out the lane long enough for Senna to stack plenty by the time she's unleashed.
P.S. Nasus's stacking is an exception; it doesn't scale with Yuumi, because it doesn't add AD. Yuumi will add Base AD, but Nasus won't give her much, as he builds low AD, and his skill will have no impact on the transaction.
Yuumi/Veigar doesn't offer that much utility, because by the time a Yuumi starts riding him, he's already 1 shotting if he's good. Amping a champ that's already amped is a rather moot point.