I've seen both in full. I'd say that the new series is never as bad as the old series at its worst, but also never as good as the old series at its best. As someone who like Urusei Yatsura, the new series was a decent watch, but it hasn't replaced the old series for me.

My biggest complaint about the new series is that it never seems to really manage to convey that sense of chaos that's central to Urusei Yatsura's comedy as well as the old one. My second biggest complaint is that untz! untz! untz! BGM they like to play during fast-paced scenes (especially early on) that to me doesn't really match up with the show's timing.

I also generally prefer the old voice acting. Now, the new series has an all-star cast, and they're all great voice actors that do a good job embodying the characters (though Kamimiya Hiroshi as Ataru took a while for me to get used to), but I think the old cast was more willing to go all the way for the sake of comedy, whereas the new cast seems more careful about walking the tightrope between exaggerated expression and breaking character. This comparison video someone made does a good job of demonstrating this, I think. That said, I can definitely imagine some people preferring the more naturalistic acting of the new cast compared to the more theatrical and stylised performance of the old.

I do like what the new series did with Lum's hair. A kind of combination between her iconic green hair from the anime and the oil-like refraction her hair is implied to have in the manga. And just in general, the colour palette is very pleasant.

I watched the 1st season (remake) and a few episodes of the original.

The new looks like a compressed version. Some newer eps have stuff that happens in two episodes in the original but keeping it as a single episode (not like 2 in 1 episodes).
The artstyle is different
The humor was updated but I'd say similar.

It feels like how the total content compares between a manga and the anime adaption (meaning some stuff or stories were cut)