-5/+10 feats
It's very easy to negate the -5 part so tends to double or triple the damage.
Negate it? There are ways to mitigate it, but you never negate it. If you have advantage, it means that you're starting at a higher baseline.
Take a reckless attacking barbarian at level 2, 16 STR, using a greatsword, and figure a nat 9 or better needed to hit.
Without -5/+10, we have 0.84 * (7 + 3) = 8.4 DPR
With -5/+10, we have 0.5775 * (7 + 13) = 11.55 DPR
Reckless attack is one of the most reliable ways to mitigate the -5, and there, the -5/+10 is netting a 37.5% increase in DPR. It's good, yes! But double or triple? Where are you getting that? I guess maybe the idea is that at low levels you are also more likely to drop a creature with that damage and therefore get a bonus action attack out of it?
As for SS, the obvious candidate is a fighter with archery style.
Without -5/+10 we have 0.70 * (4.5 + 3) = 5.25
With -5/+10 we have 0.45 * (4.5 + 13) = 7.875
which is a 50% increase in damage. I'll take it, sure! But it's not double or triple (and even with the feat you're doing less damage than a melee barbarian without it; just at range). And yes, sometimes cover would have come into play, and if you're dealing with 3/4 cover the -5 is completely negated (well, more accurately, both characters are paying it) and you're getting a "free" +10. But that's not all that common a situation IME.
Compared to getting a free bonus action attack with ability mod, without having to commit to TWF style, neither -5/+10 feat is as good.
Warcaster/resilient
Concentration rolls. Concentration spells tend to be better than ones without it.
I understand what the feats are for, and I recognize that they're good feats, but why are they particularly problematic at low levels? Sorcerers have Resilient CON automatically. Does that make them OP at low levels? War Caster is better than Resilient in Tier 1, but... at low levels you have relatively few spell slots, so you're not likely to be concentrating as often as you are later on. Is it because the DC is so unlikely to be more than 10 at those levels?
Healer feat
Compare with how much healing a life cleric can do.
Yeah, it's a lot of healing, and for a very cheap resource. That wasn't one of the ones I questioned, btw, because it's clear why its effect is magnified at low levels (like HAM and Inspiring Leader). But full action healing is overrated, simply because of the action economy opportunity cost. A thief with the Healer feat can use it as a bonus action, making it better, but not until level 3. It's a good feat, but it doesn't break anything; especially since you can only use it once per character per rest.