QuickStatements is absolutely an excellent tool. However, when using it, I found that the tool returns many errors when running my batch for the first time although my batch does not involve editing deficiencies. I had to repeat my batch many times to have required results. This problem becomes more important when using Run in Background option. When seeing last batches for QuickStatements, I found that most of the people repeat their batches more than three times to have their information included in Wikidata. I ask if this can be solved.
The reason is overloading of the WD backend, therefore throttling of update rates of any particular user or tool.
I had similar issues with adding 1.5M "Worldcat Identity" identifiers, together with "source: <VIAF ID>".
QS has to edit the whole item to add a couple of statements.
If the first statement (adding identifier) times out, then the second one (adding its source) fails and reports an error.
This issue is not easy to fix.
And people resubmitting their batches exacerbates the issue because it then leads to duplicate statements, eg see https://www.wikidata.org/wiki/Property_talk:P7859#avoiding_duplicates