Hi there, thank you for the kind words.
With regards to the +10 and +5 to primary and secondary checks at 10th level, on average that relates to 80% and 55% chance to succeed against the default target of 15.
If this were for a saving throw, it isn't too dissimilar to B/X which has saving throw chances between 50% and 70% for the average level 10 fighter (A level 10 dwarf would have a 90% chance to succeed a save vs death).
Attack bonuses are largely similar too, a level 10 character with STR as primary (e.g. a fighter type) would have +10 to melee attacks, which is close to the base +7 of a B/X level 10 fighter. Similarly a level 10 thief, with STR secondary would have only +5 to melee attacks, identical to B/X.
Skills-wise, a B/X level 10 thief would have an 85% (5/6 chance) to succeed at most thief-y things, this is pretty similar to having a level 10 DEX primary character in FORGE.
In general I find that characters in FORGE are ever so slightly stronger than their B/X counterparts, however not noticeably so.
In play testing, we have occasionally had a character make it to the higher echelons, at which point they are definitely expected to succeed at most of the things they attempt.. however I fully expect a Dragon or similar high level creature to make short work of them should they throw themselves into combat without proper planning! :)