As far personal experience, the WhatsApp web-client is pathetic except that you can easily type from your computer.
#1 Since your phone must be connected to the internet all the time you intend to use the web-client, it makes no sense to create a web-client and then necessitate your phone being connected to use that web-client. The data is consumed both on PC and device.
#2 The data usage on your phone is tremendous. As per my observation, the data usage on your phone is ten times more when you are using the WhatsApp web client. This happens because WhatsApp data is not stored on cloud but directly on your device. So every time you use the webclient, the WhatsApp data stored in your device is transferred from the device to the computer on which web-client is being used. This creates unnecessary data transfer and usage thereby increasing the date usage on device by several folds.
#3 There is no option to send videos from the web-client.
On the whole, its just Pathetic.