A. They have OK integration with Amazon (But NOT great), Faulty integration with eBay, Worse that bad integration with some other important platforms.
B. They will rob you to training, which will be done by people with little knowledge in ecommerce and sometime, little knowledge of their own platform's capabilities.
C. They will never refund your money if they break a promise, or there is a built-in fault that caused you lose money.
D. Their fees are way too high vs. the promise of "grow your business", which is an empty promise (I would love to pay more fees for business I currently do not have, which is the reason we moved to them).
E. The Support team is simply NVG (No Very Good): They do not understand their own new integration solutions, they do not understand the marketplaces they integrate with. And they will not get out of their way to understand the issues.
And the final straw: When there is a fault in one of their integration, they say: "Nobody ever complained about that"