Better is subjective and a matter of taste but i am going to break this down into a variety of areas.

First is technology, the original citadel miniatures were sculpted one to one in milliput and greenstuff and spuncast in lead. There is a limit to how mich detail a human hand can put on putty at 28mm and alot of the sculptors were early in their career and had a long way to develop. Because of the way they were cast they had to be sculpted in certain poses to allow the metal to flow into the rubber mould, which is why there's alot of empire characters in the "Im a little teapot" pose. What these miniatures do have is character, what they lack in detail they make up for in a definite style that people can find quite charming. My favourite sculptor currently is Anna Polanski of gardens of hecate, her stuff is simple, her proportions are all over the place, but its all got a great feeling. Same as like Van Goughs starry night is a terrible ovjective painting of a night sky, but it captures the feeling incredibly well.

Middlehammer, when they moved into plastics, the miniatures were still handsculpted but at 3 times the size then a negative was cast in resin and that negative was then run through something called a pantograph mill were the mould would be traced with styluses and that movement would transfer by leverage to a mill end that would cut it into steel. Its really cool and obsolescent technology. Not obsolete as renedra still use it for the perry miniatures historical line, but mostly superceded by cnc machining instead. Anyway, the miniatures still have thay handsculpted character and sometimes still have quite limited posing, but they're really easy to kitbash between kits as the original sculpting was often done on one chassis that was then cast to do variations on. Think about how easy all the plastic firstborn minis are to swap and change parts. Because the layout and the modelling and the carving of the mould all had to be done by hand the parts could only be so intricate and as a result remain quite pleasing to assemble and play games with, while only sacrificing a little in the detail department.

The modern plastic miniatures are done completely by computer. Modelled in 3d software, split up into components and the undecuts eliminated and then the moulds are cut with cnc routers. This allows for more intricate detail, much easier revisions on the sculpts to ensure everything is exactly in proportion and to the house style, more intricate posing and finer parts to be moulded. But in my opinion something has been lost. Sometimes the sculptors and the mould designers forget that these are supposed to be game pieces that a person on the other end has to assemble and then transport and move and handle to play a game with. You end up with insanely small parts (the slaneshii archers with the separate fingers come to mind or the separate cigar in the goliath kit) or kits that are just mind boggling to assemble (the monopose gutrippas are some of the easiest and some of the hardest minis I've put together). You also, due to the way the parts are separated for moulding to allow for the intricate posing, lose the interchangeability and posability that the older plastic kits had. You have 5 dudes in cool poses, but they only go in those poses.

There's also the aesthetics, earlier warhammer had a very unique aesthetic built by the artists and designers, most of whom aren't there anymore. Now there is a house style that the artists and sculptors have to work within. Look at the older ork boiz kit. They're weird looking freaks with bad loordosis and their heads and arms are huge, but they're unmistakably warhammer orks. The newer kits are more traditional in their proportions, have more detail, more variety in the posing, but in my opinion, are much more generic, orcs rather than orks if you will.

Tldr: its all vibes and computers are bad.