It is not robin hoods that are the problem.
The problem is unregistered cars of any type, there are a lot of abandomed kit builds (of all makes) out there for many reasons, it is just that robin hood produced more kits per year (approx 600 per year) than almost all other manufactures combined from mid 90s for the next 15 years. So there are more uncompleted robin hoods out there than most other single make.
There are probably a higher percentage of the high priced multistage payement kits unfinished out there.
Chin up 2Bs were designed to pass IVA. SVA came allong when government realised that this was a profit center and upped prices to suit, little else changed so properly put together it will pass.