One heuristic I like is a kind of reverse Occam's Razor: When no clear solution doesn't emerge even after a huge amount of searching, it's probably because the problem actually is complex, not because it's simple.
For example, maybe cancer does have a single unifying cause that can be fixed very easily. But the millions of hours put into studying it suggests otherwise.
This seems to be generally true about most things. Only very rarely do we get something super simple like goiter being caused by the lack of iodine, or stomach ulcers being caused by H. pylori.
Interesting point, though I think we should also consider that our ability to understand diseases like cancer has historically been limited by the observational and analytical technology available. For example, it was recently suggested that Alzheimer’s might be linked to Candida albicans, a fungus that naturally inhabits our bodies but could play a role in the disease. Just as it took time to discover that iodine deficiency causes goiter or that H. pylori produces ulcers, the complexity of cancer might partly stem from the fact that we don’t yet have the necessary technology to closely observe the cells and underlying mechanisms. It’s not necessarily the inherent complexity of the problem but our technological limitations that delay understanding—and possibly a cure.
The Candida link is interesting but not proven — so in that sense it's equivalent to your cancer guy.
The other discoveries predate a lot of advanced medical technology, though. They were the low-hanging fruit in that sense. The only semi-recent discovery I can think of is that the Epstein-Barr virus causes cancer, though that does not necessarily qualify as "simple". EBV is also implicated in MS. EBV could ultimately be one of those "unifiers" that could explain multiple diseases.
For example, maybe cancer does have a single unifying cause that can be fixed very easily. But the millions of hours put into studying it suggests otherwise.
This seems to be generally true about most things. Only very rarely do we get something super simple like goiter being caused by the lack of iodine, or stomach ulcers being caused by H. pylori.