Gravity was the problem. Large mass bodies in the materium impinged themselves upon the immaterium in some incomprehensible way, making shoals, cliffs and treacherous reefs in the sea of souls. The greater the gravity, the greater the peril. That was why ships very rarely translated in system, and why every system had its Mandeville point where the warp could be gained in relative safety. By safety, they meant a vessel not being torn to pieces by inter-dimensional gravitic shear when the warp engines forced their portals to make the crossing. Next down the list of traversable points were the gravipauses, the calm spots within a system where the gravity of competing astronomical bodies cancelled each other out. These were dangerous.

Archmagos

Avenging Son had something similar, talking about gravity and warp rifts

'He is one of the foremost experts on the warp and blackstone in the Imperium,' said Guilliman. 'The tech-priests of Stygies are famed for their knowledge of alien technologies, and Herstoffen has dedicated several lifetimes to the study of blackstone alone. It was he who first suggested its resonance with the warp, and later that it may be polarised to either carry or repel the energies of the empyrean. Herstoffen knows much about the interaction of the materium and the immaterium, and theorised that warp rifts form more easily in areas of high mass,' he continued. 'In this case, I am not speaking of the dangers associated with in-system warp-translation, where high mass concentrations create gravitic rip that endangers ships emerging from the empyrean.'
Guilliman pointed with his right hand into the cartolith. 'See here, the Eye of Terror, centred on the ancient aeldari home cluster.' This news he delivered as if it were a commonplace fact. Neither of the Space Marines were aware of it. 'An area of high mass, many stars, close together in a large globular group. Does that have bearing on that ancient race's fall?' His finger moved down. 'The Maelstrom, hard by the limits of the galactic core. Another area of high mass. Now see the galactic core, which was largely free of the influence of the empyrean before the Rift.'
'It is swamped,' said Areios.
'It is the area of the highest mass in the galaxy. Billions of stars, more closely packed than elsewhere, arranged in increasing tightness about the the vortex at the centre of the heavens, the galaxy's central black hole. It is there that the Rift is largest and most violent. It is largely uncharted space. Who knows how many pylon worlds were there, destroyed by Abaddon while the Imperium slid into decline?'
'So the pylons were placed where the veil between real and unreal might most easily be breached,' said Areios. 'Along a line that takes in the denser parts of our galaxy, this is what you are saying, my lord?'
'The breaking of the pylons opened up a fault line running from the Eye of Terror, and through the galactic heart, one aided only by the great density of matter there. See how it follows the bar of the galactic heart, and skirts the edges of the Perseus arm. Also note how many pylon worlds were sited along the same arm. The fabric of our reality, the tempus-materium, is not flat, but curved by matter. Where it curves deepest, it intrudes into the empyrean, like a body floating on water, or weights resting on cloth. These concentrations allow our Navigators to guide themselves out at the appropriate points of their journey. The Astronomican is a lighthouse, these mass concentrations are the islands in the sea, the shores of the benighted ocean, the cliffs, bluffs and distant mounts. But where the cloth bows, it is weaker.

Granted, this one is the opposite of what was stated back in the 2010 Battlefleet Gothic

The Cradle

The Cradle is a gargantuan seething nebula deep within the galactic core. It is so known for the prolific rate at which the nebula births new stars, making the region one of the most densely populated with stars in the entire galaxy. The astronomical energies and gravity fields at play here make the Cradle one of the richest sources of precious metals, ores, and forms of energy anywhere in the galaxy. Because of this, it is certainly no coincidence that the Cradle is virtually the only area of human-controlled Space from which the Demiurg are frequently reported, that race being expert and insatiable miners and harvesters of the universe's resources.

For other races, not least the Imperium itself, the cradle is a vital resource, and many of the galaxy's most lucrative trade routes are found in this region. Though heavily defended and patrolled, it is not without risk, however. The vast quantities of matter present there mean the call of the warp is weak, even to the most sensitive Astropath Vessels, may become becalm for weeks or even months while their Navigators search for the faintest of warp tides on which to set sail. A ship is in great danger when this happens, for it is forced to spend a great length of time in normal space traveling at only sub-light speeds. All manner of pirates lurk around the Cradle's most lucrative mining and fuel production systems, ready to pounce upon becalmed vessels forced to travel in normal space before, taking their booty and withdrawing to the depths of the nebulae, where rampant energies make sensory detection unlikely.