EXCEPT for the fact they decided to use .NET date formats throughout their API rather than the more traditional ISO date formats. (I do like that they sheepishly apologised for this in their API docs).

Also, they are usually a few months behind releasing API coverage of new features and modules.

Also, it is a bit fiddly having to specify a 'scope' parameter when authenticating yourself for the API otherwise you won't be able to access certain data.

Lastly, I don't like the way there is almost a separate API for the AU and US version of Payroll. I get that they are fairly different beasts, and that they are actually third party systems that have been 'integrated' in, but I wish they had more commonality.

Not everything. https://xero.uservoice.com/forums/5528-xero-api/suggestions/...

Nope. Try adding a Client Group with no clients via the API - you can't. Xero have admitted that it's a problem, and basically won't commit to ever fixing it. There are another couple of examples relating to problems with the Xero API that don't come to mind right now, but indeed there are problems.

Their per minute rate limiting caused some problems for us. Pretty annoying.