Every. Single. Customization. You make, which makes so much sense to seemingly eagerly satisfy the user during implementation, will be a massive pain, forever, with every patch and upgrade and new functionality released by vendor in perpetuity, and will inevitably cause performance and failure issues eventually. And will only be getting more expensive and painful to maintain exponentially over many years.
Yes, you should customize erp for your very specific edge cases that you a absolutely need. But:
A) number of processes Bob from accounting or Fatima from HR believe are absolutely crucial and immutable and special and unicorn and mandatory, is way way higher than processes which actually are special and must be preserved. Personal inertia is huge. More often than not, special ways of doing things which are not your core business are an unnecessary cost, whether through erp or not.
This may seem like I'm a traditional grognard IT head who disregards users and their needs, but it's quite the opposite so let me clarify with
B) The threshold of customization at which erp no longer makes sense is in fact very very low.
If you actually, really properly are a special snowflake of a company and your convoluted hr or finance or pay processes are your key immutable competitive advantage, then don't get an erp. Other name for erp is cots, commercial off the shelf, which strongly hints as to how its meant to be used. With erp, customizations should be fought tooth and nail on every level,and that's a largely accepted industry wisdom.
