The main disadvantages are that they could block your service in any of these cases:
1 - you exceed the maximum limit of emails provided for by the contract
2 - one of your users mistakenly sends some emails that are detected as SPAM
3 - one of your users sends some emails that are detected as malicious (virus or other)
4 - someone around the world reports the emails of one of your users as spammer
5 - you do not perfectly respect the policies provided for by the contract (read it carefully!)
6 - maybe other cases too...
To me (with another provider, but they are all similar...) both case 2 and case 4 happened.
Of course they can warn you (you have to tell them who to contact and how in case of warnings) and you can then fix it and have the service reactivated, but if it happens you waste a bit of time.
For the rest, I would say that there are no other major disadvantages
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)