Is the "safe" part the low variable house edge?

A variable house edge sounds like a risk to me...

house edge will only be risen if your target profit is >5% of the bank roll...

Yes, currently if you bet with profit below ~ 0.6 BTC (slightly change everytime there's new invest/divestment), you will enjoy house edge below 1%

Here is a little more explanation about SafeDice's house edge:

Is it better than standard flat house edge?
Of course it is better, lets take an example for sites with same half kelly calculation:

Flat House Edge
You play with 1% house edge and max win 0.5% of the bankroll. For every bet with potential profit less than 0.5% bankroll, you will use flat 0.5% house edge. You are not allowed to bet higher amount.
Variable House Edge
When you play with potential profit 0.5% of the bankroll, you still use the same 1% house edge. For every bet with potential profit less than 0.5% of the bankroll, you will use a lower house edge, as low as 0.5% house edge. The site also allows you to bet with amount higher than 0.5% of the bankroll with higher house edge.