This is simply not true. Redmine is one of the highest quality open source tools. If that was the case, it was most definitelly related to plugins. Using a lot of plugins is pain as they don't follow the Redmine pace and soon you are locked on latest version that works for you. With more of them you must count on interactions too. I was using 20+ plugins on Redmine and it was pain to maintain. Now I use 3 that are maintained in last decade constantly and life is goood.