Estimating fees this way is not without downside. Like you noticed, it can be relatively slow to react to a backlog that's being cleared. But the upside is that our fees are very low whenever the size of the mempool is relatively constant and that withdrawals rarely become "stuck".
But to be honest, I would generally not use instant withdraws. Currently the fees for a delayed withdrawal (which usually arrives within a few hours anyways) are as follows:
Most of the time I used instant withdrawal feature from bustadice that is why I asked the question which I had in my mind for few days.Yes as you said normal withdrawal fee is cheaper and it may arrives in the next few hours but you can't say that it will happen all the time because in my experience it may take upto 24 hours at rare cases which I don't want to face to be honest.Waiting is painful.