No, the use of informants to kickstart an investigation without having that informant be in the chain of evidence is not Constitutionally unproblematic. Defense attorneys have campaigned against it for three decades. There is a complicated balancing act involved in using but not disclosing CIs, and it is very similar to the balancing act involved in using surveillance. It isn't helpful to pretend that one use is cut-and-dry and the other not.