I'm fine with all six of the orignal books, but I can see why some people have trouble after the first three. The first three are tight, with Messiah practically an epilogue, and Children following close after. God Emperor starts 1500 years later (IIRC) and with a new set of characters, which can be off-putting. Sure, Leto II is around, and Duncan (kinda), but we almost immediately dive into explorations of religion and mythology, as well as further exploration the life cycle of the sandworm and the planetary life web of Arrakis, but first off, there's apparently people who want Leto II dead, and it's a movement led by one of his descendants. And of course Leto II has evolved into a human/sandworm hybrid thing. That's some weird shit there.
It's been a while, but I believe the fifth and sixth books also jump forward in time, checking in on the mythology and how it has evolved over time. I thought it was fascinating. And I liked the further exploration and development of the Bene Tleilaxu and all that. But the later three books seem to sacrifice actual storytelling for all this conceptual stuff, and a lot of people at least need a coherent story to follow, with characters they like or at least can relate to. We don't get much of that in the later three books. It's all this "heady" stuff. Not everyone's cup of tea.