I didn't use to do research on stats and movesets in my early playthroughs, so yeah, I remember some disappointments here and there:
Pidgeot: Underwhelming moveset and stat spread. Wing Attack used to be its strongest Flying type move. Redeemed by Mega Pidgeot repurposing it as an amazing Special attacker with 100% accurate Hurricanes. But until then... basically just use Fearow (Drill Peck, higher Attack and evolves earlier).
Umbreon: Didn't know it was intended to be a staller the first time... too bad its best stalling tool (Toxic) was a TM that in Johto is locked behind the post-game. So yeah, have fun taking enemies down with Confuse Ray and Faint Attack / Bite as its strongest STAB moves.
Slaking: Yep, I raised Slakoth in my blind Hoenn playthrough, Vigoroth losing Truant made me happy that the Ability wasn't permanent. Well, I guess it's pretty obvious how that story ended.
Luxray: A physically oriented Electric Pokémon was something new when Gen 4 came out, and it had potential to be good at that, but crippled by Spark being its strongest move. Elemental fangs coverage was also an interesting niche, but it was outclassed on both areas by Electivire, which could do the same job but better with the elemental punches. Couldn't get a good physical Electric move until Gen 9...
Serperior: Lackluster movepool. It was my starter in Black, didn't have much to offer on a regular playthrough compared to the other starters or to other pure Grass types like Lilligant or Whimsicott. Very uninteresting without its hidden ability.