Since it's been around for 13+ years, there are some legacy systems and infrastructure that limit it in annoying ways.
First, the limit of 100 custom fields is so restrictive, especially when trying to capture robust data on customers through surveys.
The shopping cart is completely useless. Why they haven't updated this is beyond me as it forces companies to use 3rd party carts that have to pass data to Infusionsoft to trigger all manner of follow up and segmentation. The API is clunky and requires pretty advanced programming chops to use it, too. So if you want to integrate something that isn't connected to Zapier, you're going to pay a pretty penny to integrate it.
Their landing pages are lousy, too, forcing many to use a 3rd party app for that, too.
Finally, their throttling of emails has grown to a ridiculous level. If recipients flag an email as spam, they stop the campaign. I've had numerous clients unable to send broadcasts to paying customers because of this.