I like that it's easy to use once you get used to how things work! It can be difficult at times figuring out what features are available or where they're located within your account dashboard if there isn't much visual help for new users (or even experienced ones).
When something goes wrong with my emails going through - whether because of technical errors from our end as well as theirs OR their servers being down/slow etc., we've never been able to resolve this issue without contacting support AND waiting several days before getting any kind of response back. If everything was perfect then sure, but sometimes issues happen regardless so having some way to communicate quickly would have helped us figure these problems faster than just hoping someone will respond eventually when nothing happens after multiple attempts over many weeks.