Just to get a counter review here. I used x1800, sold it with my sc5000, and now have the x1850 with my sc6000. With both Mixers i dont experienced any issues. Especially if you are using soundswitch, the 1800/1850 with its standalone compatibility makes a huge difference in usability, than using the laptop for soundswitch.
Once, i had a Problem with my firmware on the X1800. Had to reinstall the firmware couple of times and used some other ways described here X1800 not updating, wont restart automatically - #7 by djyg102
But to be honest, if you had something like a Rane MP2015 in your hands, a X1850 feels cheap, but a DJM900 is only ok thant. But if you have the Price difference in mind. A DJM900 Nexus 2 does come with a 1000 plus price tag compared to the Denon. However, other flagship mixer are better than the Denon one, but also cost more.
Some thought, i think the next Mixer from Denon is pretty sure on its way. The X1800 launched in 2017? And the X1850 is not more than a fix for some problems and reducing the flashy buttons. So more or less we have now 5 years the same Mixer. Everyone says the Players are on Point, but the mixer does not stand against other flagship Mixers. On the Player side is not much improvement right now possible, but they can generate new revenue with a better mixer. Maybe the X1800/1850 is in a couple of months not an issue any more, for new buyers.