As it has already been mentioned there is plenty of material out there to help you out with this.
I will just give you my personal opinion.
For endurance, tempo, sweet spot, threshold and over threshold (until about 20 seconds of duration as a rule of thumb) it works great.
For sprint intervals not so much…
Also ERG mode allows me to push myself much more than I would otherwise because I can not reduce the power. I feel this is a great advantage from the physical conditioning point of view.
However, for tough intervals, mentally, at least for me, it can feel as when I am in a group ride holding the wheels for my dear life. In other words it does not give me the feeling that I am putting down the power and attacking the interval but rather that I am following fast wheels and hoping not to drop.
This although I guess teaches you how to suffer I feel is not the right mindset and is a bit of a follower vs driver question, but just purely on the mental side.
Hope this helps! Also, I am always willing to hear other peoples opinion.