It is true, I've had multiple non payouts for CANN and FTC. Small amounts where ok, but when it came to larger amounts it seemed the system would be become "broken". Now i am not hashing with a lot of power, just an rbox 2.0 and an s3 but once i was getting more than 150 CANN a day and 100 FTC a day payouts would "break".
My guess would be that the pool doesn't have enough of these coins to pay out, either because too few people are mining those algorithms or because the specific coins themselves are rarely profitable to mine.
MUE in particular seems to be one of those coins that is almost never profitable to mine, but seems relatively popular to get paid out in, and thus it just doesn't work out. For example, there's one post on the hashpower thread that mentions a 613000 MUE balance for payment, which another post calculated would take ~16 days for the pool to accumulate, assuming best-case mining rates based on past pool performance, so good luck getting paid out something like that. No idea what's with the unconfirmed transactions, but it seems to be a problem both here and on the hashpower thread, so something about the coin seems to cause issues.