Shield Mastery: not particularly useful to monks, but awesome for fighters, monks, etc.
Shield Mastery is not a free action. It's a bonus action.
This means that it can only be used once per round to knock a foe down for a fighter, but Flurry of Blows allows prone to be attempted up to twice per round (once per flurry that hits). It's also DM dependent as to whether that bonus action can be used before the Attack action for SM.
For a PC that does not use a lot of bonus actions, Shield Mastery is very nice. For someone who relies on bonus actions (like a Fighter/Rogue using Cunning Action a lot), it's less helpful.
But yes, that feat is solid because of knocking foes prone.
K is a very limited resource, that's the difference. Compare to the rogue.
Yes, there are probably 6 to 10 rounds between short rests in most games, so at second level, the Monk's Ki is once per three to five rounds. But a lot of classes have incentive to take short rests (Second Wind and Superiority Dice for fighters, Bardic Inspiration for bards, etc.), so I suspect that most groups do short rests as often as possible.
Even at 6th to 8th level, a Monk can often use a Ki point nearly every round.
At 14th level?
1e, 2e and early 3e wizards absolutely. Late 3e and now 5e wizards? Not anymore, good cantrips, rituals and long lasting spells keep the wizard going even at low levels. WoTC clearly made a concerted effort to mitigate abs lessen wizard's early weakness!
I play a 5E wizard. Sorry, but cantrips suck. Without the standard +3 damage that every other attack in the game gets, the odds of rolling a decent amount of damage is very low.
On a 4 round encounter, I'm lucky to get one good solid shot in, even with things like Acid Splash and both of them have to hit in order to do slightly less average damage than anyone else.