One issue with intraday breakout strategies on 3x etfs is slippage....you put a stop order in at 20.00, and you get absolutely punked and filled at 20.30 (sometimes)....angry, you decide to start using STPLMT orders....20.00 stop, 20.10 limit....well, on some days your stop will trigger and the market will skip right above your limit order and leave you unfilled and the market just takes off without you onboard. ....this usually happens on the best 10 trading days of the year, when you absolutely want to be in the market for a big trend day. Of course, your wonderful backtest captures none of these "real life" problems. You only find out about them when you attempt to execute your strategy in real time.
Now, TQQQ is a very liquid instrument, so the problem I described above may not exist here like it does in others, but if you trade LABU, SOXL, TECL, etc, it absolutely is a problem. Trading "size" in those etfs is about a $20,000 order...anything above that, and things get difficult on many trading days.