Certain heroes have abilities that are based on chance. Chance is perfectly acceptable for preemptive, follow-up, and counter attacks, but the fact that you can't count on some abilities makes that ability unreliable and the hero in turn becomes unreliable. Here are some examples of hero skills that are annoying to me:
Happy to See YouDeadpool is supposedly only truly phenomenal when he acquires the buff, which in turn requires him to have randomly acquired and then uses the Happy to See You ability. In any battle, regardless of whether it's PvP or PvM, the hope of acquiring the buff is a completely random and unreliable process. In the meanwhile, you must struggle along with a hero with mediocre health and damage. There is already enough chance involved in turn order, chance to hit, damage dealt or received, and criticals. There are better heroes to use than to hope that Deadpool will suddenly become awesome. In practice, I've yet to obtain the buff in any battle ever with Deadpool.
Chaos ShieldScarlet Witch can be quite a useful blaster, and I understand that all of Scarlet Witch's powers relate to manipulating "chance". Whereas is a fine example of a good power that is chance based, is not. When I need to clear my team of debuffs and heal damage, I can't rely on to do the job. More often than not, it doesn't land on the team member that needs it the most. If either the regeneration or the buff removal/replacement of were chance based while the other effect was 100% then I could rely on Scarlet Witch, but as it is, she's too unreliable for my taste.
Sacrificial BlessingValkyrie's Sacrificial Blessing is another example of a ability that is too much driven by chance. If the buff were 100% on Valkyrie and slightly lower rates on the teammates (or vice versa), then Valkyrie would be a true Asgardian powerhouse. Instead, it's another roll of the dice whether my team will be awesome or mediocre when I use this skill. Top it off with the 25% damage to Valkyrie makes the math for this skill to not work out. Don't get me wrong. Valkyrie is a great bruiser that got me into Vibranium League one season. I'm just saying that this skill is unreliable.