Yeah your right its unpredictable but its depends of your strategy and what altcoin you choose..
In my opinion or tips i think better to always use coinmarketcap and choose top 20 altcoin or top 10 altcoin and choose them because those are legit altcoin that you can hold for a long time.
Most of them however are now controlled by bagholders owning about 50K coins. They know exactly when someone just enters the market, they could just dump few and there you lose your money. an unsuspecting investor would simply buy more because the price just drop. but these big whales can simply pull the price deeper until they exit. that's how some of them play.