The best thing about mailcot is that it allows you to send bulk emails with ease, as well as track your open rate of each email address separately! I dislike how sometimes when sending an e-mail through their platform there can be delays in receiving responses from recipients (sometimes even days).
Also occasionally they don't deliver all my messages at once which causes me some confusion. However this has not been too much of a problem for us because we have had very good success rates so far