I just watched a video that might be relevant to this topic. In it, a young, inexperienced performer was playing Ukulele and singing. I noticed that she was slowing down her strumming every now and then to allow her singing to catch up. Ukulele, when played with a strumming technique, is uniquely suited for showing off such weakness in rhythm. The constant strum is very much like a metronome or drum beat, and should keep the music in time. But here, it was allowed to be flexible, slowing down to allow the singer time to get through the vocal parts that were troublesome to her. So, I think it illustrated one of the more common singing playing problems. The Ukulele needs to be selected as the rhythmic lead instrument, and the strumming measured and accurate throughout the piece. For practice purposes, it ought to be like clockwork. Then, the vocals can be added and phrased in such a way as to keep up with the strumming. Of course, once a performer gets comfortable with singing and playing, and doesn't unintentionally get behind, then artistic license to play and sing more freely becomes OK. But for beginners, basic togetherness is important.