I'm a huge supporter of long term investing and I think it's one of the only ways that the average retail investor can have an advantage over managed money who get ranked every quarter.
The problem I struggle with when implementing it is that, other than indexing, when talking about individual stocks the proof is always in hindsight and the cost is the most valuable asset of all - time. If i buy Costco at 55x forward earnings, yes after 30 years I will probably do okay. But 30 years is a LONG time to judge any investment (heck, 5 years these days is impossible to figure out). So if anything hiccups, not only do i have a poorly performing investment, but I've lost 30 years of investing that money at even a market return.
Ive invested in many undervalued names that underperformed for 3-5 years while I railed at the market for being inefficient. Then it doubled over a year or two and I felt so justified in my analysis that I ignored the fact that it was an average return for 5-7 years.