Finally got a chance to give this a spin. Initially I had issues installing the Itoo Importer into my version of Unity, but after some tests I managed to get it to work.
Here's some results from installing the Itoo Importer into various Unity versions:
2019.4.3f1 - standard pipeline - success
2019.4.3f1 - HDRP 7.4.3 - success
2020.2.0b7 - standard pipeline - Obsolete APIs. Will automatically upgrade - success
2021.1.0a8 - standard pipeline - Exception occured inside BeeDriver: System.IO.FileNotFoundException
2021.1.0a8 - standard pipeline - copy the auto converted scripts over from 2020.2 - success
2021.1.0a8 - HDRP 11.0 with Raytracing - copy the auto converted scripts over from 2020.2 - success
So seems with 2020 there was an API change and the Itoo scripts were automatically converted. Weirdly with the latest Unity alpha version, it did not auto convert the scripts and didnt even warn about obsolete APIs, it just failed. I managed to "fix" that by simply copying over the auto converted scripts from 2020.2.
Thats probably something you guys can look at. You'll need 2 importers. 1 for Unity 2018 and 2019, and another for 2020 and newer.
I have not yet tested the ECS version of the importer.
Anyway, so on to the next thing. In max I made a simple max plane and on top of it stuck a forest using 3 different trees from the starter pack library. About 85 trees in the forest object. Exported that using the Itoo Unity exporter, and imported it into may latest Unity version "2021.1.0a8 - HDRP 11.0 with Raytracing". This works LOVELY!!!
In this case one of the trees had a weird rotation though, but its not the .forest fault. I imported the resulting fbx back into Max to see what the issue is, and sure enough, I found my 3 trees as expected, but weirdly the one had a weird rotation in the fbx! I'll do some more tests later when I get a spare moment. For now, see the attached images: