Use of you tube videos in a church service
There are two sides to this question:

1. Does the owner grant permission to use it in the church service?

Before using a video in a church service, message the owner. Usually they will be happy to let you use it, and on occasion they will direct you to a higher quality download without ads. In general, YouTube is free to play.

2. Is the church okay with the ads that are likely to come along with it?

Youtubers make their money off of advertising, and they have no control over the ads that are likely to be presented. I recall a time when a children's church program (tee-totalling denomination) started with a beer advertisement because that what was shown with the video we had selected.

Further Considerations

If you are live-streaming or recording services for later playback on youtube, you have to pay extra attention to permissions on copyrighted material. Your CCLI license does not cover streaming, but the CSLP license does. Hillsong blocks re-streaming of their content, as do major networks (nbc, etc).