string theory - Problems with Relativisic Quantum Mechanics of single-objects

As done in several text on string theory, you can quantize the worldsheet action to get the quantum string. But first, they start with the quantum theory of a single point particle by quantization of a worldline action theory.

So, I want to draw a parallel between them. For the point particle we promote position and momentum to operators fulfilling ($\hbar=1$):

$$[x^\mu(\tau),p_\mu(\tau)]=i\delta^\mu_\nu$$

Also we have the Klein-Gordon equation which arise as the first-class constraint $p^2+m^2=0$ associated with reparametrization invariance of the worldline, yielding free particle momentum states $|p \rangle$.

As we learned in QFT courses this theory has many problems, such as negative energies, negative probabilities and causality violation (which are the main motivations they state to develop another theory, field theory).

Is it the case that the theory developed by QFT authors is different and they do not know how to get the correct (and consistent) quantum theory of a relativistic single particle? I think there should be a good reason why they say that one-particle relativistic quantum mechanics is no good. Also, note that with the usual BRST procedure we get a hamiltonian $H=\frac{1}{2}(p^2+m^2)$ and according to Polchinki's book page 130:

The structure here is analogous to what we will find for the string. The constraint (the missing equation of motion) is $H = 0$, and the BRST operator is $c$ times this.

So again we have the Klein-Gordon equation and $p_0<0$ states are in the state space too. I wanted to stress this point because there is no difference whatsoever in the method you want to use (could be the old covariant quantization or BRST quantization for example).

Now, we do the same for the string. We now have the operators satisfying $$[x^\mu(\tau,\sigma),p_\mu(\tau,\sigma')]=i\delta(\sigma-\sigma')\delta^\mu_\nu$$ and we can costruct a state space and so on. But the books never talk about the potential problems that could arise such as:

1. 
   

   Negative energies

   
   


2. 
   

   Negative Probabilities

   
   


3. 
   

   Causality violation

   
   
   

So, do these problems arise in the theory of a single string in the same way as arised in the relativistic single particle?