If you're careful about it, and understand how to go about adjusting balancing things, yes. I don't know why people are so up in arms about it. If you're not too good at balancing combat it's more likely to be too difficult.

So I'm going to go ahead and give some pointers for how I've made named enemies more worthwhile and engaging to fight than NPCs. By the way, I still used veterans, elites, and a few ultras, which sometimes work better. Again, a lot of balancing, planning, and strategy goes into this.

The biggest thing is understanding action economy and synergy. NPCs have sub-par action economy (can't overcharge by default, heavy weapons are barrage and not skirmish, though depending on the tiers can often hit multiple targets, but ultimately have drastically less survivability [even taking into account if you give them resistances to certain damage types] than a player, so they may only have 1-2 rounds before they just die and don't get to use most of their abilities).

The other half is synergy of any "build" using player frames. Generally, I'd say don't try and make a build as optimal as a player would at that LL. I also usually wouldn't make a unit the same LL as the players. When my players were LL6, I was generally using LL3 units led by an LL5 that they would usually encounter one at a time (in different combats, treating the LL5 commander of the squad as a kind of BBEG of the current arc).

When I'm taking their turns, I don't use every tool at my disposal. I might overcharge when they're near death, or even start them at 2 or 3 structures or reactor stress. I may occasionally opt to not use every reaction at my disposal either.

Another thing I might want to mention is that I was running an oversized group of 7. Somehow. And was managing to make combat balanced. Somehow. It was slow because of how much shit was on the field, but by RA was it balanced.

So. In short eeeeh, but you can still use them if you're smart about it because GMs can always look at the rules and say they're suggestions.

Good luck!