Like most Pokmon cards, the value of Ducklett depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (White Flare)
When Veil of Summer was first printed, it had a big impact on the format, with people even calling for it to be banned
Thrasios takes the best parts of Tymna, our number three, and dials all of them up to 11
If you want to play a longer more mid-to-lategame style deck in Mono Red that interacts more with the table, well, it gets more challenging because it makes Mono Red's inherent weaknesses more glaring: subpar card draw and ramp, subpar answers to creatures, minimal options for enchantment removal, commander protection, and recursion, things that get more pronounced as the game progresses