Your vendor will disappoint you
I follow politics as a hobby. A past publisher from a political opinion magazine I read would tell his new writers: "Don't fall in love with any politician; at some point, they will disappoint you."
I think the same advice can be given about your AMS vendor: "Don't fall in love with your vendor; at some point, they will disappoint you."
Let me be clear: there are many reputable vendors in the AMS space. And many that I deeply respect and have worked with, some for over two decades.
But the reality is that it's simply impossible for most vendors to meet 100% of the needs of a given association. So at some point, your vendor is going to say "no" to you and you're going to be disappointed. Prepare yourself for that.
Like all things in life, it's a matter of managing expectations. You can like your vendor. You can like 'em a lot! Just don't fall in love...