The way I look at them is they are crude, badly made and the synthetic pads they come with from new are terrible, although durable. They have little finesse and are easily outplayed by a Taiwanese student sax, let alone a Yamaha. BUT, for all that, for loud rock and roll blasting they have a certain appeal.
They are also the perfect sax for a rough gig. Many years ago, in my youth, I played in some really rough Working Men's Clubs in the Middlesbrough area. A Weltklang would have been good for that use-case - a low value sax you wouldn't be bothered about getting a few knocks, and if things really kicked off, you could use it as a weapon.
(One of these gigs was at a place called The Sycamore Club, now thankfully long gone. There was at least one fight every time I was there. It had a balcony around the main drinking area, and on one memorable occasion, a drunk jumped from the balcony onto a table below, crushing it and sensing glasses and bottles flying. He just walked off. And, as they say, the band played on).
I agree with most of that.
I would start with their biggest positive attribute: Tone. They just sound really good. Big, fat, dark, tons of cojones. Very German-toned.
I would also say this: not ALL of them are 'crude' in precision. That was really variable. As a very GENERAL observation, I have found that the ones from the late 70's-80's get janky, the body metal is thin, the key castings are not well finished, occasionally the tenon walls are so thick that they would be impossible to ever expand should a tech need to tweak a neck fit. Things like that.
With that said I have also serviced a few 80's ones which were fabricated nicely. There was not slop in the keys, everything fit well, and they held regulation.
The older ones (again, generally speaking) seem to be more robust in construction.
Intonation on them are generally OK, I cannot recall one where there were notes which were wildly off. They had intonational flex like other horns of their era, maybe a tad more flex than say a JK or Kohlert. But nothing a player of vintage horns couldn't deal with.
A LOT of them are Silver Plate, and honestly the plating jobs on those I have worked on ( a good number) had held up pretty darn well....
The pads, yeah, the factory pads. In oine sense I appreciate the fact that they went synthetic, I consider it a sorta prototype experiment into things more people are experimenting with today (although I realize the intent was probably not to groundbreak, but rather cut production cost) But honestly...any Weltklang being played today should have by now had their factory pads replaced a long time ago.
Key action/response was also variable...again if the quality of fabrication and assembly from a factory is gonna be consistently variable over 20 or 30 years, some will roll off being respectably decent in most categories and some will not....I have played plenty which really felt as good under the fingers as any Conn or King or Buescher second shelfer of its time. But have also had some which needed a LOT of key attention just to get it acceptable in feel and response.
I DO agree, really, any DECENT Taiwanese, Japanese, or even some good reputed Chinese-factory student or student-intermediate Altos or Tenors can "outplay" one using most people's checklist. Even a rock-solid-reputation vintage student horn can.
Which is ultimately why I stopped buying and refurbing these. It became hard to make an argument for one when I could offer a Conn 50M or a Jupiter-made Vito for maybe $50 more.
They also do, for some folks, have a sorta 'cool' aspect of being an East-Bloc horn, and a brand which actually had a pretty long run from there....at that....
But overall, they became a horn which, today, my best advice is:
If someone came across one for like $100-150, or less (alto) or maybe $150-250 (tenor), or better yet is gifted one or finds one in their attic....and they could get it up to play condition with maybe $200-250 of tech work...you wouldn't have made a 'bad investment'. It will serve you pretty well.
But if one had a short list of: "I have narrowed my options down to these available ones", assuming that list is looking at say a $400+ asking price.....the Welt should probably be crossed off if there are a couple of pretty reputable brand names on that list....
Which in the end isn't much of a rave for these...but...they aren't as bad as their reputation makes 'em to be, I guess....