As of September 2014, YouTube's own keyword tool stopped working. It was moved over to Display Planner's AdWords video keyword suggestions. To use it, you need to have an Google Ads account. Just like the non-existent YouTube keyword tool, it was created to facilitate paid video and ads promotion. Which is why keywords that you will find there might not be closely related to the topic that you want to cover.
Depending on the plan, you are only allowed up to 100 domain searches a day. If you purchase their top tier plan you only get double of that. Sometimes it can take you several days to do certain types of research. This can start to get a little bothersome as we work with several clients and all of them need to use this software to make sure everyone is getting the best work that we have to offer.