It becomes diluted and unfocused. While it would make sense that having everything in one custom for the sake of convenience, as i said it just becomes too unfocused and you could end up struggling getting results because of how dense the custom is, in the manner that its so much to process, thus in a way making the custom ineffective or just take forever to get the results.
Plus with a custom you get Name Embedding, which makes the process more deeper than the standard store titles. Which means more deeper and profound results in general but also more recon. QTKS would take this even further.
Plus Invictus once had a custom with like 5 plus cores in it (Khan ST4 being one of them) and even though he was able to handle and get results and even the fact that in terms of flow factor he is probably one of the few that could even do it and be okay, he later made a custom with 3 cores and even he admitted that Khan ST4 was much more pronounced and the results were even better than the 5+ core.
IMO you could do a custom with 3 cores as I as well as others have had success with them in the past but I wouldn’t even touch a 4 core. I would rather have a 3 core custom and a 2 core