Yes, bike engines can be "fragile" after all they aren't originally designed to haul around 400+kg and yes you will need to overcome the lack of reverse, but there are a lot of companies who have already solved this. Getting the clutch balance right is also something to think about (designed to be operated by hand not a sze 10 boot) fuel consumption is also debatable.
On the plus side you do get an engine that is built to perfection, has it's own 6 speed sequential box, which you can have a padalshift for
, massive RPM, There are also gains to be had in handling, as it's lighter weight will lower the CoG, change direction easier etc etc