Microsoft first acquired Wunderlist almost three years ago, promising the to-do app would help the company “reinvent productivity for a mobile-first, cloud-first world.” At the time, Wunderlist founder and CEO Christian Reber joined Microsoft to continue leading the app forward. Instead, Microsoft launched its own To-Do app to replace Wunderlist last year, and fans of Wunderlist haven’t been happy it’s going away.

At the time of Microsoft’s To-Do launch, the company announced the new app would eventually replace Wunderlist and incorporate most of its best features. That hasn’t really happened yet, and fans of Wunderlist are labelling Microsoft To-Do a “half-cooked scrap.” Wunderlist founder Christian Reber left Microsoft in September just a few months after the new app launched, and he’s now taken to Twitter (spotted by Dr Windows) to reveal why Microsoft’s To-Do app is taking so long to get to feature parity with Wunderlist.

“There were technical porting challenges,” explains Reber. “Wunderlist’s API runs on Amazon Web Services, and should then be ported to Azure. But that was extremely time-consuming, so everything had to be rewritten.” The process of rewriting an entire app is clearly complicated, and as Reber puts it “easier said than done.” He also reveals that the original goal was to run Wunderlist for a year and then have Microsoft To-Do ready, but the porting from AWS to Azure means Wunderlist has remained online.

Wishing the To-Do team success, Reber also mentions it “was not a pleasant experience,” in response to a Wunderlist fan who isn’t happy with its successor. It’s clear that Microsoft’s acquisition of Wunderlist and its integration into the company has been messy, and perhaps even avoidable. Microsoft has been acquiring mobile productivity apps at a surprising pace, and the company successfully turned Accompli into its popular Outlook mobile app. Microsoft also acquired Sunrise to shutter it and move its features into Outlook mobile.

Wunderlist’s complications do sound all too similar to Microsoft’s struggles in overhauling Skype’s backend infrastructure. Microsoft has been using its own Messenger backend for Skype services for years now, but it was a rocky transition. Skype has also been redesigned so many times it’s hard to even recall the number, and if Microsoft isn’t careful with To-Do then it could end up with a Skype-like experience that’s desperately in need of being fixed.