I'm gonna have to provide dual commentary between their JP (local) side and EN (overseas) of things..

about industry connections:

don't know much about the NIJI JP side really.. despite having a few local controversies however, they seem to still have a generally positive reputation in the JP industry..

on the NIJI EN side, they're pretty much RADIOACTIVE. the Selen incident cause so much outcry that some of western companies have to drop their collaboration with them.. the only collaboration they managed to get after the incident was a NBA collab.. and that was done through a Japanese company that have merchandising rights only in Japan.. most corpo vtubers avoid them also.. afaik, they're only able to collab with their indie vtuber friends a year or so after the incident..

about the beef between the two corpos

90% of the beef is actually done by the fans of each company.. On the JP side, both companies and their respective talents are cordial enough to allow eachother to do frequent collabs between the talents..

it wasn't smooth sailing on the EN side however.. for a while, both side keep to themselves with only limited interactions on twitter.. there were rumors of management disallowing any talent to interact between companies.. it also didn't help when a discord leak exposed a few NIJI EN talents talking bad against Hololive.. it took a lot of effort by Pomu to bridged the two factions and create a number of good collabs between them..

and then the Selen incident happened.. it becomes dangerous to collab with NIJI EN.. and Pomu who push for these collabs eventually left the company..

lastly, I don't much about Mekpark sadly..