Yes, reliable engines: that would be nice. Unfortunately no
motor gliders have Honda engines; second best might be a
Stemme with a proper 4 stroke?
However the Stemme (S10) is something of a heavy weight
with handling to match: enjoyable yes, capable yes, but if you
could choose to fly a modern 15m or the Stemme you'd choose
the 15m if handling pleasure mattered to you.
I hope the 15m class has a future; I think it's one problem is
wing loading when an engine is on board - hence all the
18m/20m/21m/25m stuff with engines. So you just need to be
a purist and fly "proper" gliders, I used to do this and it is
great up until the moment you have to put your sparkly new
$100K+ carbon fibre ship into an unknown field or some dirt
strip in the middle of a desert. Then you can suddenly
understand why most of those new German ships are bought
with engines.....

As any owner will tell you: there is no such thing as a reliable
motor glider, absolutely all their engines can be guaranteed to
work sometimes, often when you actually need them

Dave