First, Horizons flora don't actually have variants, not the same way as Odyssey does. Sure, they all look the same but with different colours, but their spawn requirements are quite different, even up to requiring different regions. See also the names: we have Roseum Brain Trees, Viride Brain Trees etc, and not Brain Trees (Red), Brain Trees (Blue) and such. It's not just a matter of naming, mind.
That's currently correct, although I believe they were thinking about including Horizons bio's but they are harder to predict.
They aren't harder... on non-atmospherics. The legacy Horizons flora are quite easy, although annoying in several ways that the Odyssey ones aren't. The problem is more about the Odyssey thin atmospheric planets, and why the old flora appear on them far more rarely. Either it's a bug that they can appear at all, or it's a bug that they don't appear as frequently as they should, or the rarities are actually intentional (not sure why they would have decided that way though) and some of the hidden variables might be used as a hard requirement.
Plus there are plenty of differences in how Horizons bios spawn in the first place, which would be annoying to solve. It can be done, but it's a decent amount of work for a niche feature that's not used anywhere else.
There's also that the spawn area centers for sinuous tubers haven't been nailed down precisely (it's difficult to do, and the candidates for centers all seem to have been picked by Frontier at random, there's nothing special about those systems), so there would have to be some errors there. Precisely determining them for all tuber areas (something like fifteen) would need a lot of exploration to be done, for a tiny pay-off. Or folks would just have to settle for estimates, which would at times be wrong, and that would make the plugin(s) appear to be less reliable than they are. And of course, the "weird" differences between Horizons flora and Odyssey flora on the same body could lead to too many false positives on the former.
In other words, it's a decent amount of work to be done, and let's not forget that the Horizons flora are significantly less interesting than the Odyssey flora. There are some exceptions at least, such as where some of them appear on Odyssey landables where no (or few) other bios do, like anemones on the extremely hot thin silicate vapour atmospheres. Last time I checked, this specific example had only one found to date,
this one. These exceptions make for good photo opportunities at least.
