I don't know that its considered a "design mistake". It is not without flaws, but it does make it extremely simple to implement. Its a very acceptable point as a tradeoff between simplicity and performance. Will certainly love to implement NFS4, or even SMB3 at some point though.
IIRC the statelessness of nfs was largely considered a design mistake, and there usually are a large number of hacks to overcome the consequences.
So much so that nfsv4 moved to a stateful protocol…