Two schools of thought on this. One is that steady speed uphill is just a matter of weight, gearing, and power. But that just doesn't fit with everyone's experience of how a good road bike seems to leap uphill while a MTB grinds along.
I can tell you that my Terra Trike Rambler is a pig! It slows down hard and flexes plenty. But it's got the gears to live comfortably as a pig. For whatever reason the flex is a lot more obvious going downhill than up.
For short bursts you will often read that you can't stand up on a bent like an upright and that's true, but on the other hand you can really brace yourself against the seat and lunge hard.
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Genesis 49:16-17