More generally, you are taking on the risk of your acquirer's business (they may fail to get traction in the market, they may fail to develop their product, etc.) without getting compensated like an equity holder. This is a bad situation to be in -- entrepreneurs do all kinds of things (take VC money, incorporate rather than form partnerships, etc.) explicitly to shift risk to OTHER people. Not only are you taking on additional risk by taking the offer, but you are also not getting reimbursed appropriately for that risk.
Other issues: - You get paid more if they take longer to pay => You're going to work for a company that you hope doesn't blow up tomorrow (because you get paid more if they blow up in 4 years). => This is a dangerous spot to be in; your incentives are not aligned with theirs.
- It might be tempting to think that the acquirer could pay you with the proceeds of their next round even if their business does not take off. But $6M in short-term debt will likely scare off VCs who might fund them in their next round...