I'm more on the theory end of PL, but in my experience program committees generally regard difficulty as a negative for a paper. The main thing PCs want is for papers which make progress on important problems. Now, you can do this either by (1) attacking existing open problems, or (2) by finding new important problems.

It's easy to tell that a long-standing open problem is important, but generally these problems are open because they are hard, and consequently the technical sophistication for new attacks on them is generally very high.[] New problems can often be solved with relatively simple techniques, but it's also difficult to tell if a new problem is genuinely important or just some random thing made up for submission.

With a low acceptance rate, PCs tend to favor type (1) papers over type (2) papers, because few people feel comfortable arguing for a paper that might* be doing something important over one that is doing something important. This is widely seen as a failure mode of low acceptance-rate venues.

So over the last several years many of the top PL conferences (such as POPL, LICS and PLDI) have moved to a multiple track format in order to increase the number of papers they accept. This is precisely to give program committees the freedom to take chances on "risky" papers that might become important, or might become footnotes.

[*] Occasionally, someone will solve a difficult open problem with simple techniques, and these papers are immediately accepted with much rejoicing. It's easy to have your cake and eat it, when you have two cakes...