There are many different SEO API services out there out of which only a few would be good enough for your needs. This is why it becomes important to compare them and pick the right one that would suit your purposes.
You may have your own Idea on the kind of tools that you will want to have access to through the SEO API service and if after subscription you end up getting those that you have no need for, or you find tools that you require missing from the package then things would definitely not go too well with you.
It is therefore important that you compare different seo api services and give some of them a try so that you can find those that would be ideal for your requirements. You will want to opt for services that you can actually trust in terms of providing you with access to all the tools that you have an actual requirement for.
You should not be making long term commitments with any SEO API service without first checking how they manage to serve your needs for a short period of time as that is what would be indicative of what you are going to be getting in the long run.