I've never had any problems with any omnibus editions, I must say - and I prefer them, as I am one of those who like to read to the finish , even if it takes a couple of weeks (work intrudes rather, doesn't it ....),
But I agree something like the "Assassin" series and the 3 further trilogies, does require the odd break - Wodehouse being a perfect distraction !
And as others have said, you can leave an omnibus anytime without losing your place - unless it's a very strange reader ....
(The device, not the human bean ..
)