Generally, taking a loan to start up a business or invest is not a good idea because of the added pressure it comes with. It's more better to use a loan money to finance an existing business and the interest rate must be very low, so that while paying it back in instalment, the business or investment wouldn't have to feel much effect from it.
But as of the case of Bitcoin investment, it's a bad idea because Bitcoin is an asset that should be held in a very long time, so thinking of paying back the loan with the money invested in Bitcoin is one of the worst thing a Bitcoin investor should do, because he might be forced to sell at a loss or prematurely just to pay back when the loan debt is due, so taking of loan with the purpose of investing it in Bitcoin is one thing we should all avoid, because it's not proper.
Actually its not actually a bad idea since how could you fund your business if you didn't take a loan to make it happen? Its just you making all of things work according to what you like to happen. If you scared enough to do this then could you get a result for having that feeling? For sure not. In Bitcoin I think its the same if you could pay back the lender why not? You are not trading it, but rather you are investing it for long term so with that set up I think everything is fine.
As long as you know what you do is good and you are capable enough to pay your loans then there's nothing wrong with it. The time it became a wrong decision to take, if you know for yourself that you cannot pay your loans but you still decide to get it and used since you will would really get a big problem with that especially by the time that the lender will ask the money what you lend to him/her.
I agree with what you said. Some people do take loan to improve their business when it's facing difficulties and you will see that the business will later grow and flourish because they have where they can get funds to pay back the loan and not from the business. It's same with bitcoin investment, because this is more than a traditional business because bitcoin can give you 1000x profit compared to any business that you want to set up.
You have to understand your financial strength and weigh it to see if it will be possible for you to take loan but not from bank, maybe from your working place if you're working in a good company and pay back bit by bit without it affecting you financially. This is why I said that you must understand your financial strength so that you don't end up shooting your foot. If you think that bitcoin is a get rich quick scheme, and you decided to take loan and invest, you will do it the wrong way because you have the wrong orientation of what bitcoin is.