From the DNS client's perspective, there is a positive and a negative.
Positive: since the server with an alias record actually returns an A/AAAA, a client doesn't have to contact any more servers to get the results.
Negative: the servers for the CNAME target may be faster, relevant if the name is used beyond the A/AAAA TTL but sooner than the TTL would be set for a CNAME); or the servers for the target may be providing much finer targeting than is possibly by proxying.
