Delta's are not very good at high alpha because of the long chord length. airflow breaks away quickly and its' difficutl to rstroe hence the design has never been in favour with any designer looking to produce an agile aircraft that is aerodynamically unstable.

they are great for speed due to lower induced drag. thats why they were so popular on the MIrage III, 2000, and other variants.
In all applications where high aplha flight is required, the Delta is accompanied by a closely coupled Canard wing to keep the airflow over the top of the wing attached.

In RC models because the reynolds number is different the flow separation is less critical for normal flight but when we get down the slow speed flight it becomes critical again.

Remember something called a Diamond Dust? Thats a high speed delta, also does not fly well at low speeds or high alpha in the landing pattern because it stalls too quickly.

I've seen this characteristic on delta's low aspect ratio wings, flying STOP signs, lawn mowers, flags. Great at speed, poor at low speed high alpha due to stall