In fact, holding squats for longer is far tougher than doing the headstand!!
Above u see me do a malasana utkatasana variation .. Dharma Mittra, from whose book I decided to try it, calls it a version of the Sage Marichay(Marichayasana) pose.
This one was very exciting, at first attempt, simply because it combines the challenges of three poses: Forward bend (if you hold your feet) in the dandasana variation; a full squat ( from the druta utkatasana) and of course, the exciting balancing bit (almost a boat!).
The trick would be to hold it long. But yes, the leg will shake, u will feel this or that... but a squat is a power pose. No wonder a lot of martial arts (kalari, muay thai of which I know a bit) uses squats to power the hips from which the kicks emerge!