In short, for a highly customized netsuite environment/product i would always go Celigo.
However, we currently use the netsuite connector for amazon and after setup we have not had to think about it other than one time it had a 4 hour latency. We do a low volume of amazon sales with pretty simple products. The connector has a cheap monthly cost but dont go with their implementation team unless you want to wait -3 months for a 30 minute setup session