I've worked with them back in 2014 when they were part of GroupM, so it was really good then having their expertise to work together as we rolled out our TV campaign across multiple networks at once (Fox, NBCU).
They helped us manage all campaigns simultaneously but also provided insights from data analysis which guided how much budget should be spent where. The biggest challenge is managing budgets - too many projects can make you lose focus over time leading your account team into making mistakes or not putting enough effort towards some campaigns resulting in sub-par performance/ROI etc.. Having an expert who understands what works best online helps mitigate this risk.