Though not as nearly as portable as an electric skateboard, ebikes do offer much better terrain capabilities. It could be ridden on dirt or grass. If you have a FAT Tire Ebike you can ride on snow and/or even sand. Also, we can include various weather conditions like rain. An ebike will also go over any cracks, bumps or small obstacles on the pavement without any problem. I find that these are the key factors in comparing the two. These are also other less decisive differences.
Ebikes are much more useful if there are a couple items you need to carry. Unlike most electric skateboards, you will still be able to pedal your ebike if the battery is dead. Also, it seems as if the boosted boards are much more enjoyable especially when going down the road in between cars. It allows you to squeeze in places when needed whereas you have to ride within your indicated lane with an ebike. Finally, an arguable topic would be the safety of the electric skateboard. You may be thrown forward if the wheels get stuck or during sudden braking. Is it more dangerous than being hit by a vehicle while on an ebike? It certainly does not offer complete safety but is it any better than a boosted board.