Suggest you read this entire thread as it's all been said already.
My indy forged hollows are starting to get old, and I want to change.. What do you honestly think of Royal ?
They're great. 'Raw' / standard royals are as light as Indy Titaniums (ultra lights are great/super light), turn earlier/better than ventures, faster than Indy, and smoother than thunders.
Your forged indys are 53.5mm tall - Royals are 52mm tall, so 1.5mm difference/lower than what you are used to.
If you look at Venture and Royal, it's clear the royals are based off venture DNA, however, they're very stable like a venture yet turny like indy or ace (not 'like' them but much turnier than Thunder or Venture). Bushings are soft, depending on how much you weight and how tight/loose you ride you might want to adjust the bushings. The kingpins are just a tad longer than other trucks (not ACE) so you will have a few threads showing with stock bushings; the bottom bushing is taller than most trucks (except ACE) - this is why ACE bushings are literally the 'perfect fit' for Royals. Medium ACE bushings are my suggestion of you are below 180lbs or like a turny truck, likewise, suggest ACE hard bushings if you are over 180lbs or prefer a tighter truck.
I really want to buy the classic raws version, not the inverted kingpin. I watched the video of Ben Degros saying that it was a good compromise between thunder and indy, is that true ?
Pretty much, yes.
And I have the impression that these trucks only work with craiptail boards, but maybe that's an idea I got from seeing the guys from Girl/chocolate skate them !
That's like saying Indy are only good for Creature or Santa Cruz boards...or Thunders are only good on REAL/Krooked/Anti-Hero/There;
simply not true.And, is there a feeling of "solid" trucks or rather trucks with a sound that gives a "plastic/acute" effect ? If you know what I mean ! I have the impression that these trucks are underrated, afterward, I'm also afraid of the solidity.. Please enlighten me ahah
They are very solid, if more 'muted' than thunders.
At the same time, I'm so attached to the feeling of the Forged Indy that I don't want to change because all my sessions go wonderfully with these trucks 
If forged Indy are working for you, stick with them?