I think a lot of them are. Studies usually have two groups circumcised and uncircumcised men, but reality certainly is more complicated.
The main problem is that "circumcised" can mean almost anything from trimming the tip of the foreskin to baring the glans glans full time. What "style" was the circumcision? Was the frenulum kept, trimmed off, delta excised, etc.
Depending on style more or less inner skin is removed which can effect any number of things such as how certain infections enter the body to various aspects of sensitivity (although the effects on the latter are probably grossly exaggerated in popular culture).
Frenulum removal (and the extent of it) certainly changes how the sensation differs from the uncut state.
If insufficient amount of foreskin is removed and the patient is left with partial coverage some of the medical benefits such as no longer having an enclosed space for smegma to accumulate and decompose are negated, so the health benefits certainly are different from having a proper full circumcision. On the other hand the penis may still function kind of like an uncut one, so the sensation may not be like what a fully circumcised person would experience.
All these have significant impact on both changes in sensation and various medical benefits.
Yet a large number of studies just categorize men into circumcised and uncircumcised groups.
Conclusions usually depend more on who ordered the study and what their agenda was (if they were trying to prove possible benefits or harms) rather than on reality.
I am sure creating various studies keeps a lot of "circumcision scientists" and "intactivism scientists" gainfully employed at the taxpayers' expense. They probably even need each other the same way the defense industry in the USA and in the Soviet Union needed each other during the cold war.
Just don't expect that science will settle the circumcision debate any time soon......
