There are painful, business impacting limitations with the SLM portlet due to it being written in Flash. Larger UIM installations can experience the web GUI for SLM crash often when trying to delete records or load large data tables in the web UI, making the UI effectively unusable in terms of managing data, and preventing the ability to delete or merge any data. It can occur that you have more rows than SLM can currently effectively handle. There are workarounds for some needs the SLM fails at in this circumstance, such as using MSSQL Management Studio to perform large deletes (that would crash the SLM portlet). And origin changes can be done manually with an UPDATE statement to S_QOS_DATA without harm, but for merging QoS, while there are scripts found in the communities to do it, there is no supported workaround.
Please update/rewrite the SLM portlet in HTML5 and make it more capable and robust.