Requires it to be face-up making it very vulnerable because of low stats.
Requires your opponent to activate a card effect to activate own effect.
Monsters that it can special summon are not very powerful.
Special Summonable Monsters
Here is a list of monsters you can special summon with Soldier Dragon’s effect. If Black Dragon’s Chick or Totem Dragon survives until your next turn, you can use them to summon stronger monsters. You can special summon Troop Dragon to take all your opponent’s attack and maybe give you tribute fodder during your next turn.
Yeah, like I said before somewhere else in this site, I'm puzzled why the anime promotes Speed Duel with Skills in MR4 format alongside Link monsters, yet this game, outright called "Duel Links", doesn't use MR4 format with Link monsters.....
There's no reason to add Links until there's Synchros and Xyz to actually have a purpose for the bottleneck mechanic of Links. Fusion monsters are rarely ever used in a regular enough consistency or high enough quantity to bother having a bottleneck just for them.
Makes me want to build a deck around soldier dragon and troop dragon.