The biggest threat for people who use morphine/oxycodone in 2019 is fentanyl and analogues of fentanyl. These are a problem because they do not give the same satisfaction as other opioids, causing people to take more because "it feels weak". But they do cause the same breathing rate problems as other opioids and taking more can be fatal. The other problem is that it's possible for the tablets to contain 2-10 doses of fentanyl or another drug, because it's so cheap.
Of course, we can only know the components of the part of the pill that we measured. If we test half a pill and the pill is not even, with the other half containing fentanyl and the half that is tested does not contain fentanyl, we would not detect it.
Mixing within a single pill tends to be quite even, within a batch is usually mostly even (hopefully) and within different batches there can be big differences. In reality, most people test one part of each batch.
There is also no harm in using a simple reagent test like the froehde reagent to check if there is any morphine/oxy present, or if it could be a completely different drug. Fentanyl strips will not tell you this (of course!).
Hope this helps.