Every objective always triggering an alarm, and there are no alarms that can be disabled is a problem. There should sometimes be some way to disable at least basic alarms. Maybe a puzzle of sorts, where the penalty for doing it wrong would simply be that the alarm would trigger?
Also the default timer isn't very attainable on all but the earliest levels. Finally, the guard helicopter is always unkillable AND nearly impossible to outsmart or otherwise avoid it once it has locked onto where I am. No matter how long I stay hidden, it hovers nearby, and sure, it circles around and may be facing another direction, but it doesn't take long enough to go my direction again for that to be a useful way to evade it.