Back in the late 2000s I had my second encounter with the FBI (my first was talked about in another comment I wrote earlier today) I was hanging out with the world's leading web spammers and in those circles the 'replace words with synonyms' method was known but considered too low quality for web spam. Works for scientific literature though, where standards are lower.
Some journals are serious, some journals are pay to win, some journals are a clickbait list, sometimes it's difficult to classify. Anyone in the area know the journals and know the research groups and know whom to thrust. If you are outside the area, it's a big problem. I'm completely clueless when someone post papers in other areas here. (Not even the name of the university is a safe indicator.)
This reminds me that 20 years ago if you wanted to be "famous" you could buy 30 weekly minutes in cable tv, probably at Sunday 3 am in channel 247. (Or pay more to get a better spot.) So you can claim you were a tv star! Nobody took them seriously, and there were somewhat confiable metrics to count the number of viewers.
The problem is to define what a serious journal is. It's a hard problem. Every year we have to evaluate applications of T.A. in "fair" way, and how to count the papers always almost start a civil war in the department.
My understanding is that this is the case with MDPI. For the most part the actual "journals" that publish more than one issue are about as good as other journals. The special issues are mostly trash.
