Barcelona's history, this century, players bought STRAIGHT from South America:
2001: Cf Saviola (Arg), 36m back then. That is like 100-150M today. So, he was Dembele/Mbappe of that time.
-- stayed 3 seasons. Never reached potential. Never had a career after Barca.
2001: Cm Fabio Rochemback (Bra), 9m back then. That is like 30-40m in today's market. So, let's say Arthur.
-- stayed 2 seasons. Average career after Barca: Sporting Lisabon and Middlesbrough.
2001: winger Geovanni Deiberson (Bra), 21m back then. Around 60-80m today. Something like Brasilian Lemar today.
-- stayed 2 seasons. Average career after Barca.
2002: Cam Riquelme (Arg). 10m back then. Like 30-40m today.
-- stayed 1 season. Very good career at Villareal after that.
2008: defender Henrique (Bra). 10m, like 30M today (Lenglet)
-- played 0 matches. Played for Leverkusen, Santander, Napoli
2009: Cf Keirrison (Bra) 16m. Like 50m today.
-- played 0 matches. Poor career after Barca.
2014: Rb Douglas (Bra). 4m.
2018: Mina (Col). 10m
2013: Neymar (Bra). Paid probably 800 millions in the end, by Barto
So, we bought 9 players straight from South America in this century, without European experience.
Only 1 out 9 guys, Neymar, made it here and in Europe, but he was already one of the best players in the world.
Another interesting fact, not only that other 8 guys failed at Barca, none of them EVER played as a starter in any other big European club.
So, basically, all these players turned to be a tier of midtable La Liga teams, or at best, players for Roma, Everton and Sporting Lisabon.
In general, Brasilians were good for Barca, but mostly Brasilians who were proven on a worldclass level and in Europe like R9, Rivaldo, Ronaldinho.
And look, all these fails looked good back then.
Except Douglas.
Mina was the best defender in SA.
Saviola was the next Maradona.
Geovanni was the next Figo.
Saviola at U20 in 2001, and we bought him because of this for 100+ Millions:
I didn't measure other stats, but not ALL percentages for success are the same.
Older players are also a risk, but a success rate for young players is lower. Especially for young players coming from out of Europe.
So, roughly, let's say that a success rate for already proven players is 30-40-50%.
While a success rate for unproven potentials is probably 10-15%, so 1 out of 10 or 1 out of 5-6 at best.
So, yes, both can fail.
But chances for Thiago/Coutinho making it here are way higher than for Rodrygo, Arthur, Vinicius or even Dembele. He at least has 2 European seasons behind him.