The stuff about permit types not having good enough stability is rubbish, if made as a general statement.
There are various things which affect whether IFR certification is possible. I don't know much about this but it's stuff like sufficient provision for conducting lightning strike currents. Gliders have had their control cables melted by strikes - this is not useful!! There is a long list of stuff which has to be complied with; much of it is probably out of date or debatable but some is sensible.
The reality is that a clever instrument pilot can fly what I would call "imaginative VFR" and - ecept for not being able to overtly fly approaches around Europe, or depart into low cloudbases - get away with it. This is what a lot of the "lightweight category" crowd undoubtedly do and this is what I would be doing if I had one. Many are fully IFR equipped; they can have great glass cockpits, 3 servo autopilots, you name it.
In the UK this isn't an issue because of lack of enforcement of obvious breaches of VFR rules, so people depart "VFR" into OVC005. This is fine if you have the kit and know what you are doing. Having the legal certification allows you to do it abroad. So it's no use looking at the CAA - their hands are tied by ICAO CofA requirements. The US Experimentals can fly IFR in some cases but they can fly only in US airspace....