Recently just built and deployed an enterprise (on-prem) offering for my startup https://commando.io.
The single biggest problem by far was ripping/replacing 3rd party SaaS services that we use/pay for with native code. While building a SaaS it is wonderful to use 3rd parties to save time and complexity. Examples include Stripe (payments), Rollbar (error tracking), Intercom (support/engagement), Iron.io (queue and async tasks), Gauthify (two factor auth)... Howerver, when you go to on-prem often times the servers don't have a public interface, so your entire application has to work offline.
2nd tip. While it may seem like a good idea to create a separate branch in your repo for on-prem (i.e. enterprise branch), this is bad idea. It ends up being much better to just if/else in the master branch all over the place:
if(enterprise) {
// do something
}