Taking a loan is not a good idea because the market is volatile and there is no guarantee how it will turn out to be. Take the recent example. Imagine those who invested in Bitcoin by taking a loan. Today, they are facing losses and also have to pay their debts.
They are not really facing loses only if they can hodl but it seems impossible because it is expected to use the profit made from bitcoin to pay back the loan. Taking loan to invest in bitcoin is a big parasite that even if you tried to hodl the bitcoin, you will be in tension and panic to quickly pay debt. Its never nice to take loan for investment.