It's easy to set up campaigns with good results if executed well - we've had some great success using this platform in our agency work as an end-to-end solution from creating content through nurturing prospects via email until engagement occurs (or not).
We can't say that there is anything specific which has been difficult or frustrating at present but I think they are working towards making things easier/more intuitive so perhaps these will be addressed soon? Some of their support team seem very young compared others out here who have many years experience under them; also sometimes when contacting customer service my queries go unanswered even after waiting several days before replying again myself directly by phone call etc., however do keep trying because eventually someone may pickup :) Have used other products like HubSpot >"outbound" platforms where tracking sales pipeline was relatively straightforward though less effective overall due too lacklustre response rates amongst contacts made within those systems vs ours.