Full Idea
Although the theory is materially adequate, Tarski thought that the T-sentences are deductively too weak. …Also it seems that the T-sentences are not conservative, because they prove in PA that 0=0 and ¬0=0 are different, so at least two objects exist.
Clarification
T-sentences say " 'p' is true iff p "
Gist of Idea
The T-sentences are deductively weak, and also not deductively conservative
Source
Halbach,V/Leigh,G.E. (Axiomatic Theories of Truth (2013 ver) [2013], 3.2)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.6
A Reaction
They are weak because they can't prove completeness. This idea give two reasons for looking for a better theory of truth.