I grew goatsbeard under a lilac once. The lilac was so thirsty that nothing would live there, not even goutweed. The goatsbeard grew and flowered, but never achieved its true potential. Oddly enough, in the adverse conditions, it was somewhat running, so I did get several more plants. It persisted there for several years until I found a better place for it. It flourished upon transplanting.

FWIW, I have Goatsbeard growning near a Walnut, Oak, and Hickory. It's in it's 5 year and just started to take off last season. Suz

I planted a goat's beard this year under a walnut tree at the back of the garden. It's rather shaded and only gets moringing sun in full. The upper leaves have a distinctive sun burn look to them. Only a few minor branches have died and needed to be removed. The under branches seem to be doing well and remain green, although it is not growing very quickly. As it is so anemic there is not even a hint that it will bloom. I keep it moist but not wet.

Do they just take loads of time to establish? Anything I can do?

I've read in a few places that they're not the shade plants they're made out to be. I've had one in the shade for four years and it's still less than a foot tall and has never bloomed. I've decided to move it but I haven't decided where yet.

The ones I have seen that are huge with beautiful blooms are at least five years old. These do need some time to fully mature. Mine are in their third season, and I only got 2 tiny blooms on one plant this year (I have three together.) They were under a Norway Maple the first two years, but I had it taken down last fall. I have to admit they have grown a lot this year, but whether it's due to the removal of the Maple, or their maturity, I'm not sure. They now get half day sun, due to nearby Oaks.

Mine is under the dappled shade of a Black Walnut tree is in it's second year and seems to being doing well there. But I agree, I think they do take some time to get fully established. Also they like moist conditions, so a little extra water wouldn't hurt due to it is probably competing for water with the tree

I have one in part shade on a moist bank. It is in its 4th season and bloomed nicely this year. In July, I bought another and placed it in its pot under a Norway maple to see how it would do in that shade spot. After a few weeks, it was still looking healthy, so I planted it. It has stayed about the same since then as it probably is focusing on growing more roots.

A side note...
One thing I've noticed about Aruncus, is that the vegetation seems to get more brittle after flowering. Branches appear to break more easily than early in the season, and if branches get broken, they don't grow back until the next spring.

I bought 3 several years ago. One went under a triple canopy of Tulip Poplar, Maple, and Dogwood in a dry area. It is still hanging on but isn't very big. One went on a west facing wall in an area that some years doesn't seem to dry out but is under the canopy of a cherry and doesn't get much sun. After about 4 years it grew to a nice size and looks good. The other went against a north facing wall, again in an area that stays wet, and by the 3rd year was thick lush and 5 feet tall. I do have to use a peony ring to keep it upright, however. I put in a swale late last year to drain the area and it still looked good this year. That has been my experience, anyway.

In my experience bright, open, duppled and moist shade is the best place where you could grow them.
One in such location under the canopies of 60' tall, but limbed up oaks become 3+ footer from bare root division in 3 years and bloomed first time this year.
Another two in much more shaded location under low branched styrax barely reach 2' in the same 3 years. Never bloomed.

There are male and female Goats Beard and the male has spectacular flowers The female not so much ..I have them growing under pine trees with dappled sun and they do quite well They take a couple of years to get established...

Here is a link that might be useful: Good Earth Landscape Institute