Does this make sense, or am I missing something?
You are correct. This is a fundamental problem with all *MPPS pools. When the balance is negative, miners know that their payment has a good chance to be delayed. It doesn't matter if the pool pays recent first (RSMPPS), strives for equal payment for all shares (EMPPS) or pays all unsaturated shares every round (SMPPS). And, since this should quickly cause the collapse of the pool, "delayed" really means "gone forever".
Equalized SMPPS is ESMPPS, not EMPPS. Regular MPPS is not "vulnerable" to pool funds running out, because you have your own private limits that you are responsible for maintaining.
No matter what reward method is used, when a pool is down to zero buffer on a long round, it must either underpay or create debt for the pool owner. That is an inevitable fact of pooled mining. Nothing can stop it. *MPPS attempts to mitigate it by offering to make up for the loss later.
CPPSB is logically a combination of RSMPPS and SMPPS: When/if the pool reaches 0 buffer, it starts "discarding" the oldest shares in the round into the "extra credit" buckets, while maintaining full PPS value for the most recent and future shares (this to maintain the best possible deal for current miners, so they don't leave). When and only when it has extra funds on short blocks, it will then pay toward the "extra credit" buckets indiscriminately (backpay), until it is all given full PPS reward, and then start filling the "buffer" bucket.
CPPSEB is similar, but tracks unpaid shares rather than indiscriminate unpaid value totals, and when it's giving backpay tries to distribute it so the unpaid shares are all paid off an equalized amount.