Should I worry about the SaaS vendor going under?

Less than you'd expect. Most real SaaS MLM vendors have been operating five-plus years with profitable run rates. The bigger risk is platform stagnation rather than disappearance; a vendor that's still around but no longer shipping meaningful updates is worse than one that goes through a clean acquisition. Mitigate by negotiating contractual data export rights up front (CSV or JSON, all distributor and transaction history, within 30 days, no fee), getting these in the contract rather than the sales conversation, and reviewing the vendor's release cadence before signing. Vendors releasing meaningful platform updates monthly or quarterly are the safer choices.