Troublesome ?
On first introduction I would have agreed with you but Theilert have the engine more or less sorted now.
You must also remember that the major problem that Theilert had with the engine was a result of corrosion issues that were due to electrolytic action between the cylinder head and the hear exchanger that the airframe manufacturer had fitted.
Theilert converted aircraft did not have these problems because Theilert had control of the type of heat exchangers fitted.