My September 2017 visit to GP Gliders

That's also my understanding. It probably is no problem in the USA if the FAA accepts them in the experimental category.

However, in Europe, that means you can't cross a border with them, unless you get an exemption from the other country.

About certification, the EASA exemption rule states:

8. A Member State may decide to exempt from this Regulation the design, production, maintenance and operation activities in respect of one or more of the following categories of aircraft:

(a) aeroplanes, other than unmanned aeroplanes, which have no more than two seats, measurable stall speed or minimum steady flight speed in landing configuration not exceeding 45 knots calibrated air speed and a maximum take-off mass (MTOM), as recorded by the Member State, of no more than 600 kg for aeroplanes not intended to be operated on water or 650 kg for aeroplanes intended to be operated on water;

(b) helicopters, other than unmanned helicopters, which have no more than two seats and a MTOM, as recorded by the Member State, of no more than 600 kg for helicopters not intended to be operated on water or 650 kg for helicopters intended to be operated on water;

(c) sailplanes, other than unmanned sailplanes, and powered sailplanes, other than unmanned powered sailplanes, which have no more than two seats and a MTOM, as recorded by the Member State, of no more than 600 kg.

The funny thing here is that there is a minimum stall speed for aeroplanes, but not for gliders...

In reality, not all European states are considering all the abovementioned aircraft as exempted from certification. France, for example, is more restrictive than Germany in this respect, with more stringent limits for MTM, because the ultralight movement in France fears to lose the very supple system they have (no national certification, no medical, possibility to land and take-off anywhere with simple authorization from the landowner, etc.)

Germany on the contrary has much more restrictive national rules on the use of ultralights, with a (limited) national certification and no taking off outside of an airfield, but I believe they apply these certification exemptions to their full extent, at least for ultralight aeroplanes and helicopters. I'm not so sure about the pure gliders. So you have no guarantee whatsoever to be able to use such an uncertified glider outside your own country in Europe.

Now if you live in a big country, that is perhaps no problem for you. But I live in Belgium. The downwind leg for my airfield is over French territory....