The problem with leveraged etfs is that volatility causes a parasitic drain on your fund. Even if an index stays the same value, a leveraged version of that index will decrease in value over time if there is any volatility at all. This is a consequence of basic algebra.
If any investment goes up by a certain percentage, x, one day, then goes down by the same percentage the next day, it will end up lower than where it started. This is a consequence of the mathematical fact that
(1+x)*(1-x) = 1 + x - x - x^2 = 1 - x^2,
which is less than 1 because of the term -x^2.
If the investment stayed the same, it actually went down some lesser percentage, y, on the second day, where y is exactly the number so that
(1+x)*(1-y) = 1 + x - y - xy = 1
But now if you double the percentages x -> 2x and y->2y, (through leverage) the investment actually decreases in value, because
(1+2x)*(1-2y) = 1 + 2x - 2y - 4xy
= 1 - 2xy
which is less than 1 because of the term -2xy. Here, we used the fact that x - y - xy = 0, which follows from subtracting 1 from the from both sides of the previous equation.
For example, here is a table of a fund that keeps going up and down, but ultimately stays the same, compared to a 2x leveraged version.
Day | Original fund | % change original | 2x leveraged fund
1 | 100 | | 100
2 | 105 | +5% | 110
3 | 100 | -4.76% | 99.52
4 | 105 | +5% | 109.48
5 | 100 | -4.76% | 99.05
6 | 105 | +5% | 108.95
7 | 100 | -4.76% | 98.58
You can see that the leveraged fund goes down in value over time, even though the original fund stays the same.
You can still make money if the original index goes up, but you can easily lose all of your gains from a bull market if, at a future time, the original index simply stays the same for a while (or worse, if it decreases).