This. SAP has no restrictions on how much you customise it - you can customise and extend to your heart's content. The problems come during upgrade - SAP has very mature tooling to manage upgrades but a customer who has veered too far from standard will need to rework their customisations to fit the new version.

Many many customers have been burned by this over the years - being too trigger happy and customising/building their own extensions rather than using the out-the-box functionality. Packaged software exists for a reason, don't use it like a PaaS to built your own solutions.

SAP is now pushing customers to "keep the core clean" and stick to standard as much as possible, which is definitely a move in the right direction.