Primaris lore was fucked from the word go.
If you're not already aware, Roboute Guilliman was brought back to life at the end of 7th edition during The Gathering Storm event, which was a set-up for the status quo change of 8th edition. Coinciding with the release of 8th edition was a new range of models for Plague Marine and the Space Marine armies, which included the Primaris Marines.
Basically, at the end of the Heresy, Gulliman concocted a contingency plan in case Chaos ever decided to launch a massive invasion of the galaxy again, and had his tech-priest buddy Cawl make a legion of better Space Marines. Gulliman then launches a crusade across the galaxy with these stronger, faster and more genetically stable Space Marines, created from IMPROVED gene-seed made by Cawl.
This fucked with the established lore in that the technology needed to makes new gene-seed had been lost for millennia, and that attempts to alter genetics or improve the gene-seed to purge it of mutations had not only failed but was considered tech-heresy. So the fact that Cawl, even with his access to pre-heresy gene-seed stock and technology, hadn't died, failed to improve upon the Emperor's work, or even been stopped for his obviously heretical shenanigans for 10,000 YEARS was not only contrived but almost unbelievable.
The "improved" gene-seed also lacked many of the established mutations found in the SM chapters of the day, which essentially removed any flaws or unique quirks that each chapter possessed biologically and mentally, reducing the Primaris Marines to just bigger Ultramarines in different colors. It didn't help that the Marines lacked many customization options that matched them aesthetically to the chapters they were from and made them boring to look at, (see the recent Psychic Awakening release with Ragnar Blackmane and see how thoroughly unwolf-like the Primaris Space Wolves look as an example).