If you do:

// remove first roster item
auto first_item_jid = jabber.rosterManager()->roster()->begin()->first;
jabber.rosterManager()->remove(first_item_jid);
jabber.rosterManager()->synchronize();

// this code will crash since iterators are invalidated by 
gloox::Client::connect thread
for (auto roster_item : *jabber.rosterManager()->roster())
{
        // simulate spending some time on each roster item
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
}

The for each loop will crash even if it does not do any write operation.
And based on https://camaya.net/gloox/faq/#threadsafety, it should not?