They send you PDF invoices with no consistent formatting.
There's never a decent way to get an electronic invoice.
Any electronic files you do get, such as lists of invoices, are in a completely different format from all the others.
There's no way to place an electronic order without using their website (manually or by scraping)
At best, stuff comes by email. Often, you get emails that say you can go to the website to manually download the PDF.
Do you know of a QuickBooks-scale accounting system that does EDI easily?
But here in Poland the government apparently found a solution to this problem - it's called KSeF and it will force all local entities to send invoices as XML through their portal first (there will be no other legal way to invoice, because the KSeF number which is generated by the portal, is required for every legal invoice, without they won't be able to deduct VAT).
Only time will tell if this will bring new era of electronic invoicing or just another failed government project.
As for whether or not it will work, I think as long as Polish companies need to invoice non-Polish companies and vice-versa, trying to standardize in this manner will be challenging.