Probably that Google is dragging their feet releasing Pixel kernel and other source code. LineageOS has many years of experience getting a working system on top of bad or incomplete sources, including getting kernel source out of vendors in the first place.
I will never use GOS as long as it requires me to buy a Pixel, on principle, because it's made by Google. It's like having to buy a Microsoft Surface in order to use Linux.
You can use an older pixel, thus not really giving money to Google, and also preventing that phone from landing in a landfill. Without all that Google and carrier excess junk on board, an older phone is fast.
You can buy a new pixel, install GrapheneOS, and laugh thinking about how you're denying the enemy the OS level tracking they wanted with that device.
No, actually, you don't. And Android is not closed source. Google's add ons are.
When you buy a used phone, it's already been bought by someone. The profit is already in Google's hands. Loads of old phones are in drawers somewhere. Once Graphene is on it, it's faster, not bloated, and you don't need Google anything.
The reason why GrapheneOS only supports the Pixels is that the other manufacturers are not trying to get their shit together and release a phone that is reasonably secure. It's not that GrapheneOS supports Google, it's just that the other manufacturers are worse than Google.
If a major manufacturer released a good smartphone that GrapheneOS could support, they would get new users from the set of people who want GrapheneOS. I would gladly buy a non-Pixel as long as it can run GrapheneOS.
Which means that in a way, if you buy a Pixel and install GrapheneOS, you give more credibility to GrapheneOS, making it more interesting for a different manufacturer to consider supporting them.
What are these "huge problems" caused by Pixel devices?