The glider guys would always suggest a forward slip. It's a lot of fun to do. It's not taught often enough during primary training for powered airplanes.

Yes, but that also doesn't get any energy back on descent, quite the opposite, that is "riding the brakes on your way down"

Well it's not all lost otherwise it'd be a stall spin accident caused by performing the maneuver with too little airspeed. And that's hard to do. It's a noisy maneuver, the air slamming against the fuselage makes itself heard. Once performed it's not easily forgotten.

More dangerous than inadvertently spinning with too little airspeed is the possibility of shock cooling when relying on a forward slip for too much altitude loss. It really does need to be well-controlled.

I thought this topic was about energy gained and lost during a go-around. If velocity was V and altitude was H before the go-around, and velocity and altitude are again the same V and H after the go-around, then it follows that all the potential energy that was accumulated during the go-around (from converting fuel into altitude) has been dissipated (lost). Otherwise V would be higher the second time.

The subtopic changed from energy gained during descents to descents in general

Aren't low-speed slips something that makes planes flip upside-down when not used very carefully? (Inadvertent rudder changes corrected with opposite aileron resulting in a snap roll.)

A cross controlled stall can result in a spin (which is probably what you mean by flip upside down). The rudder changes aren't inadvertent, they're intentionally opposite the aileron input - the goal is essentially to fly somewhat sideways, so the fuselage induces drag.

In general forward slips are safe, but yes you have to make sure you keep the nose down/speed up. There's little in aviation that isn't dangerous if you aren't careful.

You're probably thinking of a skid, which is when you put too much rudder in the same direction as the ailerons. Then the lower (and slower because it's on the inside) wing stalls first (and goes lower still) and away you go. Often when turning to land, so there's not enough altitude to recover.

Yes, being that one is cross-controlled they must be used very carefully. It's really obvious that one is cross-controlling. It's the only time outside of really powerful crosswinds that you see what's below and ahead of you out of the side window. That view is what makes it fun.

100%. You are correct on that. You can’t use your kinetic energy to go around after a landing attempt.

But not because “you don’t get the energy back”. (As recursive suggested about a downhill bike ride which is the part i am disagreeing with.) You do get it back, but because you want to land you bleed it away to drag. And once it is bled away you don’t have it anymore.

So we don’t disagree about the practical implications for flying. I’m disagreeing with recursive’s particular statement about downhill cycling and what it implies about the physics of the problem.