There are some known issues and bugs in the sotadata mapping which will be addressed when the person in charge - Andrew VK3ARR - can find time to do that. At the moment, yes, some QSOs do appear to “go missing” in the sotadata map, but in time those bugs will be ironed out. It is what it is, it costs us nothing at all, and we have to live with its’ imperfections. Like life, really…
Another difference between the two systems is that the SMP map has potentially more data on “non-SOTA” participants/QSO partners’ QTH positions available to it, since it uses a different DB. In particular, the DB table which the SMP map uses for QTH positions can be updated (via the activations page) by any user logged into the system, so missing data can be added, and faulty data can be corrected. That’s a feature which I am fairly certain will NEVER be included in the sotadata system.
But if QRZ.com is not an option for the lookup of locators another option is hamqth.com.
As far as I know it is not as restricted in number of requests per day.