This sentiment is one of the annoying side effects of social media culture these days. Namely, several prominent social influencers on YouTube (which kind of emboldens the blogger world) who regularly comment about the MP5 being "archaic" and "totally outclassed" by newer offerings like the SIG MPX. It's popular for some like that to dunk on old/mature platforms for the entertainment value. The problem with such broad statements like that is there's never any nuance discussed. Also, they usually don't clarify what the application is.
I liken it to people arguing about pistol performance and differences in shooting splits, for a defensive pistol. Platform A (maybe a striker or SA gun) makes it a little easier to shoot sub 0.20 second splits. Platform B (maybe a DA/SA) might be 0.05-0.10 seconds slower on average. They argue while completely ignoring the subject of shooting at assessment speed. In essence, maybe 0.25-0.35 second splits is assessment speed for most people (I'm making this up, but probably in the ballpark), in which case both platforms are more than capable. The question then becomes, what other capabilities does the platform need to meet? Maybe you need a trigger that's forgiving enough that it's not a liability (e.g. DA/SA or DAO vs a competition grade 1911 trigger)? Regardless, it's focusing on the few percent of the situation often at the expense of everything else potentially good.
Also, often these types of detractors are coming from the standpoint of maximum competition speed, or CQB and HR speed, which is sort of ridiculous to use as a baseline. It's just not what most people are doing most of the time. Ballistic wounding factors aside, if I have a need for a PCC, I'd much rather have a platform that's mature, robust, performant enough, and flexible enough to setup for my own needs. The MP5 does that really well IMO, even if there are some minor annoyances with ergonomics, manual of arms, etc.