First, I really pity trials in your guild, if every trial was kicked out after a mistake or 2, we'd never get any raiders, people tend to get stressed on their first run with the new guild, so you tend to give them longer period of time to see if they adjust or not. That's why most guilds have a trial period of 2-4 weeks and not "1 attempt at the boss, fuck up and you're out", sorry, that's hc pugs for you "LINK CURVE BE GOOD!!! FAIL = KICK!" That's one reason why I prefer guilded runs over pugs, because I don't see this toxic pug crap in guilds, contrary to all the dudes claiming it's guilds that are toxic.
Second, nope, you can't trust people.
Some examples:
1. new trial 9/11 mythic, good logs, he dies from some stupid shit, we decide bench him on harder bosses, decide to give him another chance next reset, next reset he's a bit better, but still dies from stuff he really shouldn't, we bench him on aggramar - argus with the stance let's see next week if he performs any better, he deletes everyone from btag ninja transfers in the middle of the night with no goodbye.
2. a guy plays a healer and is benched on a boss progression because we decided to use different healer setup, he offers to play an offspec dps, raid leader tries him out because we could use more ranged, his dps is abysmal (40% less than every other dps), the the raid leader tells him we're gonna use another player for this boss and he will bench (you always have someone on the bench, you don't run exactly 20 players, it's not always "you're shit" message sometimes you just want a different class), he could just sit out that 1 boss and then go back to healing... nope, ninja transfers without saying anything to anyone, since then I counted he had 7 transfers on his wowprogress history (he left us at the end of tomb), so yeah typical player of the kind "if I get benched I leave"
3. an applicant has good logs and good guild history, says he was in some higher guilds but wants to play a bit more casually now, we try him out, the moment he gets cutting edge he suddenly "has to go away for a week", we're like ok, no probs, some people have trips and holidays we'll wait for him to come back, he's 3 days overdue according to his notification, next time he logs in, insta leaves the guild without a word, I swear that was one of these "I just wanna someone give me cutting edge then I'm quitting until next expac", and I bet he won't have much issue finding a guild, unfortunately
4. there was a guy who was declined from several guilds for being "a guild hopper" but he swore he just had bad luck and he's not leaving for no reason, my guild gave him a chance, came for 2 weeks, then never logged in again, never heard from him a word, so yeah, "bad luck" there mate
5. an applicant insists to be tried cross realm when cross realm mythic is enabled so he can see if the guild is worth transferring for, after 3 weeks he passes his trial and transfers... then vanishes for 3 weeks straight, doesn't log or anything, after 3 weeks he immediately transfers back with no explanation or goodbye, fucking logic there you pay for transfer and then you stop playing?
We had people who left with a reason like "I'm going to play with my old friends" or "I don't wanna raid anymore, demote me to social, I'm gonna focus on pvp" or the usual reason these days "guys I'm gonna faction change to horde" and I'd say any day I have more respect for people who say a word than those who don't and left you wondering wtf happened. Still sucks if you lose a person, but at least they have the decency to say goodbye, majority of them don't.
Any guild leader except maybe in super top guilds would prefer a consistent and reliable average player over a "great performer" who's gonna "burn out" after 3 weeks, but you cannot know it. And it happens. More than it should.