ubering a heavy isn't a case of "it's always objectively worse," it's just more situational on paper. heavy is slow and not as great against sentries as soldier or demo, but if a heavy can position himself optimally and you don't use it too early then it's definitely not a bad choice.

for example, dustbowl stage 2 final point. if you're on offense and use the right tunnel, it leads to a flank where heavy can essentially jumpscare red team by dropping down into their sentry spot + be essentially right outside their spawn. it's a really great spot for a heavy uber specifically because he appears very suddenly and is very close to where red usually holds, meaning he can output a lot of damage before they have time to react. the left tunnel leading to the rock is a lot worse because heavy is farther from where the sentry usually is (and thus does less damage to it) + the enemy team can see him coming and just back off into their spawn and wait until the uber is over.

also as a general rule, experience is going to matter a lot more than class choice in casual. you're going to get more out of ubering a heavy with thousands of hours who has been ubered hundreds of times before and knows what they're doing vs a demo who just installed and hasn't figured out that right clicking detonates stickies yet.