I never see the same reservations expressed in other circumstances where historians are writing after the event.
The proximity of the writing about an event to the actual event is a key issue for historians in all contexts. Specifically it requires historians to consider the source material when a commentary is written decades after an event.
Sometimes (as will be the case in current academic historical texts) the source material will be clear and referenced. But for ancient texts we are often completely in the dark as to the earlier sources used in the derivation of the final text. And in this case the weight that is placed on that text is consequentially diminished.
The issue with Josephus and Tacitus (amongst a range of issues) is we don't know what source material they are basing their very limited comments on Jesus. Was it genuinely independent, or were they simply using early christian texts that were floating around and editing it down to the bits they believed were true and relevant - namely there was a guy called Jesus, he was executed and people followed him (that's basically all we can take from the texts of Josephus and Tacitus ignoring the obvious later christian alterations).