The combat pillar feats do need balancing though. Not against the other pillars, but against themselves.
Case in point: compare Grappler and Shield Master. Shield Master is so superior to Grappler that it's laughable.
I think if one is only considering Advantage on attack rolls, there is some truth to what you say (bonus action vs. attack to knock a foe prone). And the saving throw stuff is very sweet in many builds.
But for some builds, Grappler is better than Shield Master. For example, a raging Barbarian. He has advantage on strength ability checks while raging. It's easy for him to grapple a foe and also attack with advantage on round one, on round two he knocks the foe prone and attacks with advantage. On round three, he just beats on the guy twice per round. Sure, this has a bit more of a setup, but the foe rarely escapes the grapple/prone condition and has disadvantage on his attacks back. Even a tough foe like an 18 Str Troll only has a 22% chance of escaping a grapple each round against a 5th level 16 Str raging Barbarian PC with Athletics trained. And if the foe tries to grapple him back, that foe tends to fail.
Edit note: NPCs with multiple attacks per round have a higher chance to escape the grapple per round.
With Shield Mastery, he can repeatedly knock down the foe and get advantage on his attacks, but the foe can also stand up and attack back with no penalty (or run away or whatever).
So the Grappling raging Barbarian typically gives up two advantaged attacks and in return, he gets rounds and rounds of him having advantage (without using his bonus action) and his foe having disadvantage back. That's pretty nice. Is it as great as the "no damage Dex saves" on occasion from Shield Mastery? Maybe, maybe not. But a grappling Barbarian can just decimate one foe at a time with little damage in return and no need to Frenzy.
And grapple can work well with a Barbarian that wants to switch between two handed weapons and one handed weapons (2 handed when not grappling, 1 handed when grappling). The Shield Master Barbarian is pretty much stuck in combat with a one handed weapon and a shield.
The restrain portion of Grappler is fairly weeksauce. Just knock the guy prone and he already has the same problems. The only time restrain works better than prone is when other ranged PCs attack. Personally, it is better to just Grapple/Prone him and beat on him most times, but restraining a really tough foe so that your Sharpshooter Archer can pin cushion him at -5/+10 can be sweet as well.
Grapple also allows one to move foes precisely where one wants to, so the NPC can be moved into area effect spells, etc. Shield Master can do this somewhat as well, but the PC must be able to actually position his PC and it's harder to always move the foe around corners, through other PCs/NPCs, etc. Grapple has slightly finer control.
I think that people look at the defensive portion of Shield Master and say "Wow". Yes, it is nice, but for a raging Barbarian that already gets advantage on Dex saves (and for some Barbarians half damage on most attacks), on a 28 point Fireball for example, it means that he typically takes 0 points of damage instead of 7 (instead of 14 for some Barbarians). Meh. Ok, but the Barbarian can save a lot more damage taken when his grappled/prone foe has disadvantage on his attack rolls on multiple rounds. And both feats can be combined with an Enlarge spell by an arcane PC to handle even huge foes.
Also, the grapple strategy is not often affected by initiative order. If the Shield Master PC knocks down a foe, that foe can sometimes (based on init) stand back up before another melee PC can attack it with advantage. The grappled and grappled/prone NPC rarely escapes the grapple, so he rarely gets back up before another melee PC can attack. And the grappled/prone NPC has disadvantage against all melee PCs that it tries to attack, not just the Grappler.