Yes, depending on the house layout it could take over a minute for the water to get hot. Our last house, because of its size, would take that long - even with the conventional tank.
The cold "shot" you got in the middle of the shower was because the water volume dropped below the minimum flow rate, and the burner shut off. He either needs a new flow sensor, or take the restrictor out of the shower head. This may also explain the extended turn on time.
can't try to run too much hot water at once on different sources, doesn't sustain itself as well.
Actually, the more taps you have open, the better the heater works. The increased flow is sensed, and the modulating gas valve ramps up to a higher firing rate. My daughter wakes up and showers at the same time I do in the morning. We both have scalding hot water - indefinitely.
If you have a shower right after someone, an on-demand water heater provides the identical performance that a conventional water heater will.
Not always. Once the tap has been shut off, and then turned back on, there will be a short delay before the burner re-fires. During this time, cold water will be flowing through. It has happened here.
It seemed I had less control, it would get cold for a second or to even when I was still adjusting on the hot side.
That is part of the problem there. If you are using individual hot & cold taps, the hot should be full out, and then make adjustments on the cold side.
For us, having unlimited hotwater is well worth the trade off of waiting 30 extra seconds. At the end of the day, they are not perfect, and not for every one. But saying they all suck is unfair. It is like saying all Axiom subs suck because a few have had amp issues.