as long as a pos system does not gets the same attention from security specialists as bitcoin i consider it flawed.
security/crypto is not something a programmer can get right without much training in the field

but the main problem (for me with pos) is basic-logic: i dont think its possible to secure a system with itself. bitcoin uses pow: which essentially means you need to get hardware and this hardware needs time to do some work: thats the key. its a physical key which lies outside the system.
my coding experience tells me: it is impossible to build something secure on an insecure base.