I want to start a SaaS or product based company and thinking of going with the .NET family of technologies, as I have been working on it for the past 3 years. Can anyone provide insights on the pros and cons of this approach? Would I be able to run modern JavaScript frameworks on top of it like React/Vue.js/Node.js?
You should be totally fine with .NET. You would benefit immensely if you are planning to use some of the PAAS services on Microsoft Azure. Most Azure PAAS services have a first class support for .NET.
A small advantage of going with Javascript on server side would be that you get to use/learn only one language both on the frontend and the backend.
You can totally make JavaScript and associated frameworks work with .Net. Anyway, you'll keep the client-server and different modules in the system loosely coupled. I don't see any issue with this. Write the backend using .NET and use JavaScript for the front end.