HubSpot is too rigid and the system is built so you have to conduct email marketing/marketing automation in the HubSpot way. This is fine for anyone newer to marketing automation but if you've work with any of the other good systems, HubSpot is a downgrade because you're limited in what you can do.
Emails - everything is based on a template - if your template doesn't have a module built in, it's more difficult than it should to add something.
Reporting - terrible unless you pay more advanced reporting. You'll get really good at manipulating "smart" lists to pull the information you want since basic reporting is useless.
Workflows - not enough options to customize - HubSpot is so set on the idea of 'nurturing' which is obviously part of lead gen, but not every email workflow I want to create will include nurturing. Maybe I want to create an email campaign that sends emails on specific days (e.g. a webinar), not X days from a specific date. There's also no way to connect branches. If you have any complex email campaigns, get ready to build dozens of branches because once a contact goes down a path, that's the path until they run through the workflow.