The Intel modems aren't awful in a vacuum, but compared to the Qualcomm modems in older iPhones, they were a pretty big step down. When I upgraded from an iPhone 7 Plus with Qualcomm modem to an iPhone X with Intel modem, the reception in my apartment went from 4 bars to 2 bars, and I've had a ton of issues with GPS reception over the past few years.
If I set my iPhone X in my lap or in a cupholder while navigating in the car, the GPS will occasionally cause Apple or Google Maps to think I'm drifting off course, and I'll sometimes miss turns due to that. It has to be up high on the dashboard to get usable GPS reception. My wife's iPhone X is even worse... she can barely use navigation on her iPhone and typically just uses our car's built in navigation instead. I haven't seen such poor performing GPS since my original Galaxy S Captivate. None of my iPhones with Qualcomm modems would lose GPS signal as easily as the iPhone X.
Not only that, but with the iPhone 7, there was one model with a Qualcomm modem and one with an Intel modem, so it was easy to do direct comparisons, and a ton of websites ran tests that showed that even at the same signal level, the Qualcomm modem got faster speeds than the Intel modem. If I recall correctly, I believe Apple even disabled some capabilities of the Qualcomm modem in that phone just so that one iPhone model wouldn't be significantly better than the other, but the Qualcomm modem still outperformed the Intel one.