In my opinion once you start to need heavy customization in off-the-shelf ERP solution (ie. because you do not fit in the vertical that the system is designed for) you are generally better-off writing the whole thing in house. And if you want to do it as web application then ASP.net with WebForms is one of the more productive approaches to that problem.
As someone who now works in the "backoffice" I would say that building backoffice solutions from scratch is the height of hubris if your primary business doesn't involve those backoffice functions. There's a million small things on the compliance side that need to be addressed, and which the incumbents know about and already handle.
This applies even if you need heavy customization. In fact, it applies even more--since that level of customization usually means sufficient complexity of backoffice needs that only the pre-built service providers will have the sufficient depth and scope to cover you.
