Fundamentally, combat in this game is about Offensive, Defensive, and Utility skills/abilities/traits.
You win by using your Attack+Utility resources to damage the enemy while using your Defensive+Utility resources to prevent them from damaging you.
The better you match up your defense to the opponent's attacks, the better you will counter their damage. For example, you want to use a channeled block for a long attack like rapid fire instead of wasting 2 dodges. You want to use dodge for unblockable attacks. You want to use reflect on projectiles. You want to use reveal when your opponent stealths. Furthermore, you want to use your defense to counter the MOST damaging skills and MOST critical CCs that the opponent uses because you (unless you're a mirage..) don't have enough defensive resources to counter them all. It can be good play to take autoattacks to the face while saving dodges for big hitters (eg. deaths judgement).
These big hitters are often known as "must-dodge" (or block or invuln or cleanse or whataever) skills, because if you don't, you'll probably lose the fight.
That brings us to Holosmith. Holosmith has 3.5 must-dodge skills all on less than 15s cooldowns. let's go through them:
Phew. That's a lot to defend against. It's not impossible though. With careful play, you can do it. (eg. on my Engi, I'll swap to toolkit while the holo is in mid-air jumping to me with Holo Leap, then use Gear Shield to block Holo Leap and the first hit of Corona Burst. Then I'll dodge Shockwave and most of Blitz.)
However .. you have to keep in mind Holo gets all those skills for free with its elite spec. Now that I've wasted all my defense, they can just disable forge and play like a core engineer for a short time while it recharges. Meanwhile I've got no defense left and they have a full bar of weapon skills and utility skills. How am I going to defend against Overcharged Shot and Jump Shot when I spent all my defense on Photon Forge?
I just don't see how Holo can ever be balanced with Core Engi when it gets such ridiculous power and sacrifices nothing. The heat mechanic is not a downside. In fact it ends up providing almost as much as a free heal skill every 20s.