The short answer is that no tracking service can track keywords for YouTube ads simply because video ads don’t support the {keyword} token.
According to Google, the reason for this is that the search term doesn’t target your ad, but targets a video. Then, when the video is displayed, the video triggers ads that have similar keywords to the video.
So, if your ad is displayed and somebody clicks on it, it’s because the ad was related to the video, not because the search directly targeted your ad.
There’s really no keyword to return…
Taking this a step further, suppose the original video doesn’t show your ad, but the visitor clicks on a related video, then on to a video related to that video, which does show your ad.
In this case, the original search term could be quite unrelated to your ad…
That’s why YouTube In-Stream ads don’t support the {keyword} token.
Here is a list of tokens that YouTube video campaigns do support however:
{adgroupid}
the ID of the ad group that served the ad
{campaignid}
the ID of the campaign that served the ad
{creative}
the ID of your ad creative
{device}
returns “m” for mobile, “t” for tablet or “c” for computer
{loc_interest_ms}
the ID of the location of interest that triggered the ad
{loc_physical_ms}
the ID of the physical location that triggered the click