While working with online critique groups might seem easy and host lots of different advantages, there are several disadvantages when exchanging your craft through emails.
STRANGER CONCERN WITH THEFT
With the growing issue of plagiarism, sending your precious novel to a complete stranger is cause to baulk at joining an online group. There is a risk of someone stealing your work or idea as it is very easy to copy and paste documents into a new one. Not knowing who the person really is only heightens this risk and may be the biggest con you should take into consideration when thinking of choosing an online group.
TIMELINE AND WAITING
Unless there is a specific timeline predetermined, working with people strictly by email restricts the timeframe of when you will see your edits again. You are left waiting for days and days wondering when the other person will email them back. With online groups, you also run the risk that they might never get back to you, leaving your hanging without any word.
Any writer knows how easy it is to become a hobbit when writing a book. All you want to do is lock yourself away in a room with no phone and no internet and write, write, write. But, with an actual face to face meeting group, you are forced to get out of the house once in awhile, at least. With an online group, you run the chance of continuing your hobbit ways.
Because there is no stress of how a person is going to take your opinion face to face, the chances of someone being overly harsh and mean rises. Without having real contact with another person, people tend to be a little more honest and mean more than they would be with the person sitting in front of them.