What are longterm risks with DCA into LETFs

I am still learning best ways to properly use LETFs before I actually start using them, specifically UPRO. I understand the time decay risk with them but if you were to DCA into UPRO vs SPY with the same amount, and hold 10-30 years through all the ups and downs, I don't see risk with UPRO? What am I missing, seems almost too good to be true?

I mainly just put all my money into S&P 500 and let it ride anyways, always assuming it will grow in the future and is my main investment for retirement as for most people. So if in the future if S&P crashes and never reaches ATH again we are all fucked anyways right? and if it eventually reaches another ATH then DCA into UPRO would outperform it