This discussion was actually started in 'what did you shoot' when Ona posted a 1 over par score on a Par 66.
As with all questions of 'difficulty', it's a relative term.
It really depends on the reasons why the course hasn't got many par 5's or lots of par 3's.
In Sydney, Bardwell Valley is quite difficult, even though it is a Par 62. While many of the par 4's are short, they aren't easy, and most of the par 3's are genuinely difficult. Every hole is hard if you miss the green. Kiama is a Par 66, but a really good test of golf. All the holes are 'normal' length, it's just that there are 8 par 3's and 2 par 5's.
Sefton is an all par 3 course, but again, the greens are tiny and really hard to get up and down if you miss them. The Von Nida tour standard pro's had a tournament there about 10 years ago, and I think about 8 under was the winner over 36 holes.