Just had an hour and so rocking backwards and forward....  latest update below.  I have kept the earlier stuff for ease of reference.

Our Migration Experience To Date

We are also in the process of migrating to QuickBooks online as we have so little time that moving to an alternative is not viable right now. 

We have, to date, found these issues:-

• The migration of the account balances generally appears to works fine.  We have around 20 minor errors to fix. 

BUT

• Customer jobs no longer work.  We are going to have to use Items rather than jobs going forward which is less than ideal for us and how all our other systems in the company are set up.
• There is no export function for customers details meaning we are forced to tidy all the incorrectly migrated data from customers/jobs manually.  Very time consuming and painful.  The migration has imported 6600 clients and projects.
• Item lists don’t export.  
• There are NO customer types (but there are in the American version).  Why oh why would you do this?! 

I am only a few hours into this but if people find it helpful, I will keep updating the list on here.

Update re-data transfer

• If you have ever allocated invoices/credit notes against journals, these will need redoing.
• Several discounts (we don't often use this) have exported weirdly.

Update regarding our own journey

• We have 6005 customers and jobs (never realised we had so many!) which you can only view 300 lines at at time on QBO.  Marking the old jobs that we can no longer use "inactive" on QBO is sluggish and incredibly painful.  I managed around 1500 in well over an hour.
• Tried ringing QuickBooks support to see if there is an export/import tool I could use to do this.  There isn't.  Lovely lady but unfortunately no additional advice.  Interestingly (?!) she said each QB product is different in each country.  America, Canada, New Zealand etc. She doesn't know why.
• I am now in the process of marking all our customer base "inactive" on QB Desktop and then only marking the clients on the Aged Debtors report "active" with the intention or re-migrating and seeing where I get to.
• I am also planning to do a similar exercise for our chart of accounts, marking any old used accounts as inactive.
• Hopefully when I try migrating again everything will be more manageable.  If not I will need a large gin tonight.