There is some hydrilla in Beaver not too far from the Lost Bridge South ramp, I noticed it there a couple of years ago. It doesn't appear to have spread at all in the time that it has been there. Maybe like Ham says, water fluctuation keeps it under control.
Loch Lomond, here in Bella Vista, has in about 5 years become completely infested with hydrilla, used to be none at all, and the bass population has exploded in that lake. However, the POA also manages the hydrilla, they have a mower to keep it a few feet below the surface in the shallow coves and they do some spraying around docks and boat lanes so you won't see it matting up on top like it will in lakes where it is unmanaged.
If you want to do some reading on the subject, search 'Lake Ouchita hydrilla'. I used Google, you can get varying results based on the search engine you use.
Here's somthing I copied out of one of the articles.
Hydrilla, a highly invasive exotic weed, is present at water deeps of 5 to 30 feet is a succulent rhizome type weed that cannot be controlled by lowering the lake. The explosive growth of this weed overpowers other aquatic vegetation. Essentially, density of its growth creates an environment that harbors smaller forage fish but precludes larger game fish from feeding. This creates a non-supportive environment causing the size and numbers of popular game fish to decline.