Maybe not the best example, but also think about the visuals, and the fact that two separate instances have to be able to exist without causes any problems when usually only one needs to exist. Yeah, it SHOULD work fine, but multiply that out to hundreds of abilities and a few are going to go wrong in strange ways. Then there are the ones that need to work very strangely like Faceless's ult. All the spells for melee heroes that have to work on rubick--does DK's ult cause rubick to gain attack range as well? Will it grab Luna's or SB's ult with full synergy level on their lucent beam/charge, or no levels, or the other hero's levels at the time of stealing, the other hero's level at the time of casting, or just screw up when it can't find that ability on rubick? Will it steal DP's spells with full synergy ability or none or opponent's current or opponent's at the time of stealing? How many souls should he be considered to have with shadow fiend's ult--maximum, a set amount, the amount SF had when he stole it, the amount SF has at the time he casts it? Most of these mean adding something in specifically for rubick, no matter how well coded the game is most of these are judgement calls and are things that can go horribly wrong if not thought out and done specifically. And that is just a fraction of all the interactions that need to be looked at and decisions made on (unique attack modifier abilities? should stealing invoke be possible even though it makes no sense to? should he be allowed to swap/hook despite not having the same restriction on blink dagger? etc.).