Nikara and Sam are fine. Nikara’s healing is actually really strong when you DON’T need the extra damage since pretty much any combo deals tons of damage already. Such situations are common. The Medivac cooldown even with full masteries has its limits and Rattlesnake’s healing doesn’t help vs high damage output units like Siege Tanks, Immortals and especially Hybrid. This is even more apparent on Brutations where the damage output can be really high.
Sam might not have the most useful active ability of the Mercenaries, but his AA damage rivals Tychus’ when upgraded. That’s all he really needs to be useful honestly, he brings RAW damage and his double attack fast attack speed makes him waste high HP and low HP units alike. He’s my preferred hero for gunning down higher priority targets alongside mighty Tychus.
Vega is fine. If the enemy has Battlecruisers, Carriers or Broodlords, she’ll be very useful. Each dominated enemy unit is basically a unit you no longer need to worry about. Her anti-air snare gear makes her very helpful alongside allies with weaker anti-air capabilities. Ever snare a pack of Liberators in front of a line of allied Siege Tanks? Quite glorious.
The only two heroes I ever have a problem using with Tychus are Cannonball and Blaze. Their “role” as a damage soaking tank is best filled by expendable allied units, Warhound’s Turrets or Vega’s dominated units. They don’t provide enough to be worth the hassle of trying to keep them alive.