Given the large amount of silver available in the late XVI centuries, other high content silver coins started appearing in Europe, chief among which were the Dutch leeuwendaalers and rijskdaalers. These coins had a lower nominal silver content than Spanish dollars (.885 vs .909 for the Mexican coins and .916 for the Peruvian coins) but had one big advantage over their competitors: the silver content was stable.
The big issue is, despite her well developed minting industry, the Spanish Kingdom had serious issues (or no desire) producing coins of consistent purity. This issue has never been properly addressed: mints in Saxony and Italy had absolutely no problem striking consistent coins in the XV century already. Spanish authorities were obviously powerless (or unwilling) to deal with the issue: between 1630 and 1650 the Potosì mint produced an unknown number (running in the hundreds of thousands of pieces of eight) of coins with a silver purity under .400 instead of the prescribed .916. Despite being warned many times by both the Crown assayers in Cadiz and the Genoese bankers, the Spanish government intervened only in 1650. Both the governor of the Potosì mint and the chief assayer were put to trial in Spain, sentenced to death and brutally executed, yet there were many who felt these two men were either scapegoats or just accomplices in a larger counterfeiting scheme.
Again in 1664 the chief assayer in Genoa sent a letter to both his superiors and the Spanish Crown complaining that a recent shipment of pieces of eight struck in Sevilla had a much inferior silver content than prescribed. This time no action followed.