Hi Siebe,
The Buddha has mentioned that if one sees the dhamma it is only then that one will see the buddha. So in this sense, I think “seeing the buddha and understanding the unfathomable nature of the Buddha does not refer to his bodily form. It is something deeper, like his “gathi” and “guna”. Which connects with what Vince talks about.
I’m not sure if the Buddha has mentioned that he is omniscient, but he has mentioned that what he has preached is just a minute bit of what he knows. So for all intents and purposes we can think of that as being omniscient. So yes, an arahanth will not understand all that a Buddha has understood. For example the Buddha says that only a Samma Sambuddha can fathom the intricate details of how Kamma works and for others not to try understand the details. So even Sariputta, who was second only to the Buddha in wisdom, was not be able to comprehend that.
The Buddha also says that only another Samma Sambuddha can truly see and comprehend the nature of a Buddha. So this can be taken to be the unfathomable nature of the Buddha.