I think there are two issues, one being engine reliability and the second is suitable terrain. and the two do overlap.

The Shuttleworth Collection is far and away the largest and most long term operator of old, and especially rotary engines in the UK, in the Camel, Pup, Avro 504, Bleriot, Sopwith Triplane, and other lodgers, like the Bristol Scout. 

Overall, they are fairly reliable performers, and one often sees maybe three flying together, so it is not a rarity, or a fluke. Over the years, they have occasionally faltered or failed in flight, and been dropped into the wide open Bedfordshire fields, usually with minimal damage. It helps that the Trust owns the estate, and can keep the landscape undeveloped. 

Cole Palen was to a degree inspired by Old Warden, when he established Old Rhinebeck in the 1960s, though he took the operation down a more air display/ flying circus route, with great success,  and there was a troubled period after his death with infighting and struggles.  Engineering standards have been in the spotlight, such as the Avro 504 crash ( in the trees) which had a missing keeper and cotter pin, and NTSB was critical in their investigation.  It is a fact that the operation has lost a fair number of aeroplanes, two in the last two months ,and usually ending up very broken in the trees. Shuttleworth too of course has had many incidents in six decades, but usually damage has been slight,  and sometimes no damage at all.  

Rotary engines are not dangerous per se, but they are ‘tricky’ and not altogether reliable.  They DO need proper and constant upkeep,  and it is surely self evident that to take them into the air,  requires a ‘Plan B’ in the event of a forced landing,  which ideally does not involve a densely wooded landscape.  The only solution I can think of is just hop them within the airfield boundary, or clear a number of glades, to accommodate a forced landing. Whether land ownership or funding could make that a possibility, I do not know.