The electronic repellents do work to a degree at keeping squirrels away but you do need to do some more things to fully keep the squirrels out of your yard.
You can use motion activated sprinklers that will be triggered when the squirrels get near them and the squirrels will eventually learn to stay out of your yard.
If you have a problem with squirrels eating the birdseed from a bird feeder you can put some safflower seeds in the bird feeder which are bitter in taste and the squirrels don't like safflower seeds and will leave the bird feeders alone.
You can also place safflower seeds where the squirrels are attracted to to keep them away.
The safflower seeds are not harmful for the birds so no harm will be done.
Strobe lights are also effective at deterring and repelling squirrels. Place some strobe lights in your yard and activate them when the squirrels are around.