We're doing an integration for a warehouse management system used by a few of our clients which are fulfilment centres.
We opted to use ShipStation as it supports a wide amount of platforms so their customers can easily integrate.
The issue we're facing which may force us to move to another platform is the lack of ability to send tracking information back to ShipStation for carriers such as TNT, DX and others. There should be an option to manually specify the carrier so that tracking details can be passed back through to the sales channel. It looks like the only option our clients will have is to use "Other" which doesn't tell them anything about the carrier.