Most likely, the objects that entered Romanian airspace were drones from the Shahed family or possibly Kh-101 cruise missiles. While shooting down such objects by a military aircraft is straightforward, it is restricted by numerous limitations in peacetime.
Wirtualna Polska journalist Łukasz Michalik explains that even if a plane detects the target on radar, it must visually identify it. On radar, the suspected drone could be mistaken for a helicopter, for instance. Additionally, near the border, it might even be a Ukrainian machine.
This necessitates close flight, and given the presence of Russian objects in NATO airspace for a few minutes, intercepting them in NATO airspace is virtually impossible. Furthermore, during peacetime, military aircraft cannot exceed the speed of sound, which prolongs the time required to reach the incident location.
It is also worth noting that shooting down such an object carries risks. If shot down from a close distance, debris from the target could damage the aircraft and fall to the ground. In the best-case scenario, nothing happens, but in the worst case, it could cause a forest fire or fall on inhabited areas.
For these reasons, what might seem like a weak response from both Romania and Poland to the intrusion of Russian drones is understandable. On the other hand, if a full-scale armed conflict were to arise, any unauthorised flying object in the operational area without an active transponder or lacking NATO friend-or-foe identification would likely be shot down.