Most developers resort to cheap tactics to ramp up difficulty. In general the quicker the game needs to be finished/less money for development the more you'll find cheap tactics to make it 'harder' and therefore 'longer'.
Many a good game suffers this flaw.
Uneven difficulty is sometimes excusable depending on the game mechanics. In some cases where a game has lots of different mechanics, its more just chance you find one that seems unusually hard compared to what came before and then comes after. This is just a risk with some games and its tricky for the developer to balance it.
On the other hand where the game mechanics are pretty consistent or simple, there's little excuse for odd, misjudged spikes in difficulty.
And poor use of cutscenes, checkpoints or re-spawning enemies is simply bad level / gameplay design and should be judged harshly.
Try to be reasonable... its easier than you think...
