One major issue we have, and it is partly Netsuite Professional Service's fault, is that our Location classification was renamed and repurposed as a type of custom segment prior to us going live by our implementation rep. Because of that, we do have a slightly special setup where taxes are entirely based on the Transaction Ship To address, and Location has to be ignored. We also needed line item taxes, as only some of our services are considered applicable for use tax, and the services taxable vary by state. So, we aren't really sure exactly why we are having trouble. Very likely our unusual setup is part of the problem. The other may be that we generated over 10,000 invoices per hour during our bill run. We could be having concurrency issues with either Netsuite or ONESOURCE, but so far neither company has been able to determine why we have such a request timeout rate for tax calculations.
As for outages, they are definitely happening, and in fact they had a 3 or 4 days period where they had repeated outages (either one or two months ago). Their short term solution was to add more processing power, but they said their long term plan is to move to an elastic cloud-based provider to give them the ability to handle large bursts of increased usage more gracefully.