Strength of $H(k\|H(m))$ as a MAC algorithm
Should be comparable in strength to $H(m||k)$. The weakness is that a collision in the inner hash breaks the MAC. Using strong hashes the strength in bits is $\min(2^{n_O},2^{{n_I}/2})$ where $n_O$ is the output size of the outer hash and $n_I$ the output size of the inner hash.

But since cryptoanalysis usually breaks collision resistance long before it breaks keyed modes like HMAC, it's riskier than the security loss against generic attacks indicates.