I've recently been working my way through the "Marvel Universe" cartoons from the 90s and watched through this Avengers series last week. On a scale of 1-10, I think the series scores a 5. The things mentioned above are fairly accurate, though maybe not so extreme.
1) An Avengers series without the big 3 isn't the worst idea, especially if you're a "universe" fan and get a kick out of seeing other heroes get some screen time.
2) The future setting wasn't that noticeable except in terms of the costumes.
3) Ant-Man, Wasp, Hawkeye, Vision, Scarlet Witch, Tigra and Falcon aren't the lamest heroes around.
4) The armor is terrible, especially the armoring-up animated sequences. These sequences are minimized later on in the series.
5) The storylines are original and feature Ultron, Kang the Conqueror, the Zodiac, along with the Masters of Evil headlining for the villains. Captain America, Namor and Iron Man make guest appearances during the middle of the series.
6) The voice acting was one of the stronger aspects of the show (along with the quality of art/animation). The series generally looks and sounds good. The voices for Hawkeye and Wanda could be better but they're nowhere near as bad as Luke Perry's Rick Jones (Hulk) or Robert Hays' Iron Man.
7) No comment on Ultron since I'm not particularly attached to him in any way.
8) The armored costumes are terrible and do a great deal to pull you out of the mood for watching the show. I had started watching the series a few times and was turned off by the costumes. Finally got in the right mind set and watched the show on my phone going to and from work on the train. Ant-Man and Wasp have the worst armor/costumes and Hawkeye has it pretty bad too. Falcon, Vision and Scarlet Witch don't fare as badly. Wonderman and Tigra get off pretty easy.
Compared to the current Avengers series, Avengers: United They Stand is pretty awful. On it's own, a little more tolerable.
My rankings of the 90s Marvel animated universe:
1a) Spider-Man - excellent. Loved the guest stars.
1b) X-Men - very good.
2) Silver Surfer - I wish they would have run a second season.
3) Iron Man - season 2 is good, season 1 is terrible.
4) Fantastic Four - season 2 is good, season 1 is miserable.
5) Incredible Hulk - great voice acting for the Leader, Gargoyle, Gen. Ross. Rick Jones and She-Hulk make you want to stop watching.
6) Avengers - could have been so much better
7) Spider-Man Unlimited - just awful.