The big question is WHEN you buy. If you buy in the middle of a bull run, fail to sell at the top, then the price crashes below your buy price and you're still holding, then you're missing out on the gains that you could have made if you bought later. You'd still be making profits in very long run, most likely, but it shouldn't be that hard to find the bottom or close to the bottom of a cycle. Consider this a slightly advanced for of "buy and hodl".
This is absolutely correct, but if the bolded part is truly "not that hard", no one would lose money on Bitcoin. You may want to elaborate a bit. Personally, I feel like beginners should just dip their toe in, especially if they're in it for the long run; trying to catch dips without experience would simply contribute to decision paralysis.
Also, I share this on threads like this all the time, but statistically,
lump sum is statistically better than dollar-cost averaging. Both have their advantages, but it's always good to have more info before making a decision.