Stacking SAFEs is easy because each one feels small. A founder raises one at a 6 million cap, another at 8, a third at 10, and each conversation is quick. The surprise arrives with the first priced round, when all three convert at once.
Post-money SAFEs make the dilution of each instrument fixed, but they also make it additive. Three post-money SAFEs of five percent each are fifteen percent of the company before the new investor writes a cheque, and the option pool top-up the lead asks for usually comes out of the founders' side too.
The fix is boring: keep a pro-forma cap table that converts every outstanding instrument at the next likely price, and update it every time a new SAFE is signed. It takes an afternoon and removes most of the surprise.