Taking into account the load times and the multi-step processes required for a simple request (exporting all contacts from a segment, creating dynamic content, running segment counts, and saving/deploying emails), I waste a lot of my time waiting on page loads and count updates.
Few to no systems have integrations with Epsilon so you will need your own developer to integrate data feeds into the system, otherwise you need to devote 10-30 minutes to manipulate the CSV data from your data feed to correspond to Epsilon's finicky importing needs. The software has the ability to do advanced projects if you have the time to set them up and the development resources to set the appropriate API integrations to use as a trigger.
The biggest frustration is the lack of control you have over your business and preference pages. All Epsilon Harmony forms (including your sign-up form and preference center) must be hosted on the Epsilon server and you will not have any ability to make updates as a user. Instead, you will need to pay upwards of $2,000 to make minor changes to this page, based on their time and scope estimates. Even with these scopes, it took over 12 weeks from request to completion to get a form field removed from our form. Heaven forbid we wanted to update the form fonts, colors, and images to better match our branding we likely would have been quoted well over $20,000 for the update.
While it may be the cheapest option for an ESP of its caliber, you make up those costs in ad-hoc projects or software to support a multi-channel marketing program.
If you run your lead-collection through several different sources that are not linking to the Epsilon sign-up or Manage Preferences form, you'll have a challenge adhering to subscriber preferences if you have several preference channels (i.e. Newsletter, Sales/Offers, Special Events, etc.) As they do not keep records of when someone opted out to a specific channel. Review collected by and hosted on G2.com.