I've worked with SAP in two companies: for the end user, it's slow, ugly, hard to use, and full of idiosyncrasy.

I've used two open source ERP, both as end users and as maintainer (granted, in smaller companies). Both were objectively better. Props to ERP5 and the composition it allows by the way, especially for the web pages that are easy to modify without prior knowledge (controllers and the database especially are a pain to work with however, but unless you want to create a new module yourself, you won't need it).

I do not want to belittle your personal experience, but two companies make no convincing picture, IMHO. I am working daily as a developer with ERP systems. Once you get used to the idiosyncrasy they are usually no longer slow or hard to use (unless the implementation partner has really messed up - which happens).

Open source ERP can neither cover the functional range of main ERP systems like Oracle's or SAP's, nor can you expect the same level of domain knowledge being applied to it in development (with a small fraction of the developers the large ones have, lacking the specialization at the large vendors), nor do at least I know of success stories in that regard with Fortune 500 companies, as their main, driving ERP system.