Its not broken to be tanky and deal damage
There are 3 dynamics to consider
offense defense utility
tanky champs with a lot of offense and defense can be very frustrating but they tend to lack in utility. But what does this mean??? Well to put it simply there is a disproportionate level of mechanical skill involved with abusing offense, defense, or utility.
That is why typically as long as the team isn't stupid, in one for all supports will beat bruisers for much if not all of the match unless they are actually hardcountered which can happen on occasion. Because dealing with utility is also very difficult when it is being used properly.
Ways utility is used..
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evading minion damage after harass, minions can easily deal more than ignite if you let them. Some champs have a way to deal with this others don't
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Wave freezing, some champs don't have the sustain for this or a shield
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mobility, some champs get to overcome this weakness periodically but once that's over usually they tend to be a lot weaker
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sustain in general, you can build sustain yes but that kills your build tempo and causes other issues
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lack of kit synergy, yes getting free armor pen is very strong but true damage does not scale with free armor pen and neither does building armor penetration in some cases.
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scaling, just because you can kill 1 teemo with a warmogs doesn't mean you could kill 2 with a bloodthirster. Scaling matters. Some of these champs don't really scale well with offense meaning that after you reach a certain level of tankyness they cant realistically do anything to you very quickly
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reliability of abilities, unreliable abilities can be abused for chances to engage and disengage.
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general kitability, it doesn't always mean you get to deal damage while you run. The point is that
could build 7 
somehow and he will still need more than 5 seconds to kill anyone in most cases simply because they clicked away from him. Lets see you do that against 
or 
name any champion and I will tell you abit about where they fall on the Offense, defense, utility triangle
You could also consider it to be a 3 sided pyramid if you take into consideration the unbalanceable factors such as information advanatge
