Surprise is a problem because it oversimplifies the circumstances under which opposing forces come into contact with each other and what constitutes the beginning of hostilities. Why are the bugbears hiding, do they just hide there 24/7 or are they aware of the approaching PCs somehow?
If they...
As with all these kind of questions the acid test is this... have your party fight a NPC Assassin using these abilities and see how fair they think it is