The easiest way to get from Abu Dhabi to Dubai may be by taxi, but it costs more than taking a bus, and may cost more than taking a Careem.
The cost for a taxi from Abu Dhabi to Dubai is about AED 280 to AED 350 one way, depending on both traffic and your final destination.
One nice thing about using Careem is you can know the final price you will pay before you book, which a taxi does not offer.