It won't be unstable just because of fact of using VM.
It could be unstable because of poor network, shared CPU/RAM resources on peak periods, bad storage (HDD/SSD), under-estimated workload.
The thickest argument is shared CPU resource... QS by nature of QIX engine requires full access to CPU, so it could utilize 100% of power any time and immediately - VM with shared resource will prevent this, which will lead to poor user experience.
It's hard to say with 100% certainty how bad it will be without knowing # of Apps, # of Objects in Apps, # of Users, Amount of data loaded into the system, etc.
Hope this helps