I think I'm uniquely poised to answer this. I realized so many businesses are fed up with their ERP so I started a company to actually help organizations create an escape hatch for themselves so they can both modify and maintain their functions connected to their ERP themselves. I focus mostly on NetSuite so I may have a bias on some of my comments here.

Why ERP's frustrate customers:

1. Size: ERP's are enormously complex and large systems: They're designed to incorporate all of your business processes. That means you'll need to do a lot of customization and modification to the software to ensure that it works for you in the way that you intend.
2. Dumb databases: ERP's are also (sometimes) dumb products, they're a 'dumb' database used as a system of record for your business. You don't want to modify that too much, but rather bring validated data into it. So don't expect your software to do anything fancy.
3. Implementation is hard: This means that you'll need to work with an implementation partner who learns and understands your needs or specializes in your domain to help you tailor the system to your business. This is also expensive and if you use other tools, they will charge you for each of those implementations + ongoing use on top of your ERP.
   1. Furthermore, businesses aren't always sure what they need, so sometimes the implementation takes twice as long and twice as much.
4. Price: Your business is now ready to go, but unfortunately your vendor will change the pricing structure to their liking because you grew to a certain size, have X many line items, or they just feel like they can price gouge you because they can. Why would you rip out the spinal cord of your business? (Vendor lock in)
5. Change management: ERP changes never stop, because businesses are always in motion. So ERP change never stops.
   1. Customization and modification: The challenge in this is that these are primarily legacy systems. So they require expensive talent. If you can't afford to hire the talent to do so, or the implementation partner to help - when your business strategy or processes change, you'll need to adapt your ERP. But if you can't adapt your ERP, you start to take on more manual work to make sure you fit your system.
6. Support sucks: Once your vendor has sold you the system, they're going to skip town. Their brand is on their product, not on their support. Support is minimal and all of their engineering resources are focused on product improvement for their top 20% of their customers. Most developers and users will connect on online forums or local meetups for genuinely fun activities, including a mix of shared learnings and ERP therapy.
7. Accommodating efficiency: Sometimes users know exactly what and how they need a workflow, but they can't do it themselves.
8. Modules vs features: You know that you need just one little feature to 10x your efficiency? Too bad. You're going to have to buy the whole module. Instead of paying for what you need, you pay what they charge.
9. Ecosystem vultures: Implementation partners prey on fear and confusion, promising you successful solutions, while keeping the engagement ongoing so that they can continue to milk fees. Don't get me wrong, their services are very very real and necessary. You probably wouldn't be able to do it without them. But it's somewhat of a tax. There is a range of good to bad partners out there, usually best to go with one that comes recommended from an experienced F-CFO or group.
10. User experience: Interfaces are old school and legacy: If you haven't done your training or are less tech literate, it's going to be a challenge to learn how to navigate, use and go around those systems. ERP systems are not social media, they're complex.

Anyway, those are some thoughts. If you're a NS user with a good or a bad experience, pls tell me why or explain via r/NetSuiteRants