The Tavern

Why are TSO's summoned angels/daevas and Trog's brothers in arms considered so strong? None of those monsters are particularly strong ones.

Pretty much any BIA can deal an extreme high amount of damage per turn, more than most forms of offensive you can get in a normal game. They usually are able to kill pretty much anything up Zot easily. And you get this this without training any skill or requiring item. And you usually get them before Lair.

What's to underline about BIA is that most of them aren't very tanky - the big noticeable exceptions are iron trolls and stone giants - so they may not do a lot against a strong enemy or at least with strong attacks - e.g. hydra - if rolls go bad.
The best way to use a BIA is to tank for him. For example, if you use a BIA against an hydra, you shouldn't let him between you and the monster, because he could die before dealing any damage. You should take the damage while he deals pain from the side or the back. What's more, against melee speed 10 monster - again, hydra - you can just walk back while having aggroed the monster, because the BIA is faster and will kill the stupid foe who will usually just keep walking towards you.
And another good rule to learn is: if a single BIA can't get the job done, use two or three BIA. In a 3 runes game the monsters able to survive that are countable with an hand.

I've less experienced with Angels' in a 3 runes game. In extended they'll rip anything up because they have good spells, lot of defence and deal tons of damage. 2 or 3 angels will easily kill any 1 or lord - even unique ones - especially if hasted. Honestly Considering how good are their defence - plus TSO's bonus - , I never had to apply complex mechanics while using them: I just evoke two or three, haste them and tw anything to death
Probably their offence isn't as nearly as good against tough not-vulnerable 3 runes common enemies, but I imagine they're still very strong.

Berserk goes a long way towards buffing the bros. I can't recall the last time I encountered one of those both hostile and berserk; if I was on the business end of that more often, I'd probably have a better feel of what they can dish out.

in general, monsters are much stronger on the player's side than they are on the monster's side (since the player is much stronger than monsters).

e: e.g., the player can take on any of makhleb's lesser servants in a straight fight by like, lair 1 usually. makhleb's lesser servants are useful as fuck until after elf:3.

As others pointed out, any summons that are not completely outclassed by the dude you are fighting tend to be very useful. As a player you see this phenomenon all the time; a yak is usually a pushover one-on-one by the time you enter Lair, but even yaks can give you grief if you allow an entire pack to surround you in open terrain. Same general principle.

In addition, any reasonably robust summon can run interference while you heal or buff or reposition, create distance so you run upstairs, etc., which is useful as well.

Thanks to Holy Wrath, TSO allies are only somewhat better than mighted against demons/undead (88% more damage rather than 50% more) so they are very good outside of extended too, and you don't get any junk summons unlike what BiA can give you. They last very long, and with TSO's blessings and a bit of your help, an angel or daeva can clear much of a floor for you, which makes them feel much like permanent allies. You can do this with little to no invo - hey, same as Trog - just call them ahead of time because the fail rate can be pretty high.

Am I crawling wrong? For lots of my games, yaks are kind of tough 1v1 when I enter Lair. Usually not "use a consumable" tough, but very much "even with a chokepoint, I can't take the whole pack at once." When I leave lair, they're pushovers unless my character is really bad.

But it could be that most of my characters are actually pretty bad...

I think it really is strongly dependent on the character. If you have good aptitudes and decent gear yaks are pretty casual by the time they crop up. If not, then sometimes running away is the answer.

Okay I reworded my post slightly even though the main point was (I think) clear enough.

Thanks -- I was actually curious as to whether I'm terrible at skilling because I really am kind of bad at Crawl.

I tried out the angel summon and it is nice. I got a daeva both times I tried, and it tanked a lot of damage and was also able to deal a lot of it.