Whenever I read about SAP (which I hate, so don't mistake this for an objective take) it reminds me of the discussion about self-hosting your email: "Oh, no, it's too complicated, you are better off paying someone else to do it for you". The problem being, I host my own email and it works fine , which makes me suspicious of the common takes about ERP too. Now, I get it, once you have to deal with taxes and payroll in…
It may work using an open source model. However you don't really want to invest into non-core business(es). It takes away money, talent and energy that you would better put into doing what you do best.
I've experienced this first hand developing an invoice system(not my core business). I still use it but if I were to do it again I would develop only the functionality that I was missing in the commercial offering and try an integration. Time spent on the none core business is time wasted.