Did someone had actually ever had a bad experience flying with
a telescope in the main hold (with the obvious "FRAGILE" labels on it}?

I have witnessed luggage thrown/launched onto dollies and into the hold from
the lift, fragile sticker or not. I've had suitcases crushed under so much
weight that the shells were embossed. If your scope case is well padded
inside and substantially armored on the exterior, it could survive an
aircraft hold with only a need to collimate. If the scope gets an x-ray and
tagged for inspection, you can't count on everything being buttoned up
correctly. UPS or FedEx is more gentle.