I really had some problems about the raid bosses (not raiding, on catching)

Magikarp. Been raiding these fish for ages, still nothing worth evolving, IV-wise nor shiny wise.
Found 100%s of higher raids though: Exeggutor and Piloswine.

Done about 67 normal + 313 leg raids and so far I got 5x 100% (wb Groudon(seen 29), Kyogre(seen 28), Rayquaza(seen 57) and 2x Raikou(seen 37))

I only got 1 100% IV mons from raid and it was
Groudon....

Got exactly 1 perfect mon from 450ish raids total. I felt particularly unlucky about Kyogre, had to do about 45 before seeing one that's better than 91% 14atk and it ran. Didn't find another good one after that either.

Scyther is actually the only raid boss I've gotten 100% from, ironically. I've gotten several raid bosses in the 90-96% range. My first breakthrough Moltres was 96 also

Lucky: Alakazam and Azumarill (only one raid ever on each of them, got a perfect the first time and quit doing them, only 2 perfect I ever got raiding, perhaps unlucky that one of my only perfect was an Azumarill). Gengar, always seem to get a really good one 92-98%

Unlucky: Scyther, Exeggutor and Porygon, I used to solo these all the time and could never get one >90%. Legendaries, only two ever >91% out of 105 raids won (maybe some higher ones got away)
Pinsir and Onix, I can't even find them to raid! :(

Lucky: Ttar. First raid was a 98%. Since then, not so much love, though, and low on candies. Partly because everyone around me eschews Ttar raids for legendaries.

Unlucky: Groudon. Caught 10, best are an 89% and 87%.

Lucky: Kyogre. No 100% or 98%, but got 96%, two 93%, two 91%, four 89%, from 17 catches.

Unlucky: Machamp. Caught 36, and seem to have this nasty trend of either getting adequate to good IVs and Heavy Slam or Close Combat, or poor IVs and Dynamic Punch. Most of the latter group are now going into gyms and then back to the professor for double candy - might hit 1000 Machop candy by the end of the Kanto event. And with all that effort, I have 1 98%, 2 96%, and a few more at 89-91%.

Average: Mewtwo. Have two, 84% and 86%. They work great, despite being several IV below perfect.

Average: Rayquaza. Caught 15, best is 93%, a couple more at 91% and 89%. They work great when powered up (or even not if wind buffed - have several of those). Could easily build a team of 6 at level 25 with double flying attacks.

All that said, the difference between 100% and 91% in a gym battle or raid is negligible - if one did 1000 fights, the 'weak' 91% mon might lose one or two that the 100% would win. That's why I've powered up two Groudons and both Mewtwos (the PC/SB one to level 36 so far).

490 raids total and never seen a perfect. Caught 2 98s (rayquaza and entei) that's it.

I have done 702 legendary raids so far and have seen only three 100%: Entei (ran), Kyogre (ran), and Rayquaza (caught).

Most of the others I've seen are middling to low (for a raid boss) IVs in the 71-82% range, but for everything besides Articuno, I have managed to get at least one over 90%. The best I've seen/caught of each are:

Articuno: 89%
Zapdos: 93%
Moltres: 93%
Entei: 98% caught (100% ran)
Raikou: 91%
Suicune: 96%
Ho-Oh: 96%
Lugia: 98%
Groudon: 96%
Kyogre: 96% (100% ran)
Rayquaza: 100%
Latios (so far): 98%

For regular raids, I've done 281, and the only one worth mentioning is a 98% Ttar. The rest were pretty much immediately transferred.

Two 100%ers from raids, so far; Porygon and Onix.

Dear person, you‘re talking out of my heart. I have the exact same bad luck as you. 394 hatches, 187 legendary raids, 80 normal raids, not a single 100% (I‘m 100% sure about that, lol), but also 2 10/10/10 mons (Tyranitar and Kyogre, both caught). My best are also a couple 98% from the raids.

I've been hitting raids pretty hard since they came out. 258 regular raids, 937 legendary raids. Of those, I've encountered 5 100s.
Zapdos - caught
Suicune - caught
Rayquaza - caught
Rayquaza - ran
Latios - ran

My only 100% so far is a Qwilfish I got from an egg :(

Probability of not getting 100% one out of 400 raids is about 13.5 %, so nothing special. Same as not getting desired charge move after 3 TMs.