I agree in principle, but I am sure a digital flanger with a high enough sample rate should be fine. I use 3 different flangers - a very old analogue one, an old one I built around a digital delay line and my Zoom 4040 which ain't bad (latency is pretty poor when changing effects, but it's handy;-)

The best one for actual warmth of sound is the one I built, but that could be 'cos the analogue one is very old.