Custom dev works fine with SOAP Api or with Restlets, but you should look into Mulesoft if you want to buy ready to use.
I’ve used node.js apps connected to SOAP API, node js apps connected to Analytics (odbc), apps connected to restlets. All work very easyly
Remember to check security requirements, volumnes (concurrent users) which are tipically challenges. Also remember that workflows and Forms actually affect SOAP API behavior. Besides that its all straightforward.