I think you should remain skeptic of EVERYTHING!!!
But, that concept aside...bioperine may offer some increased absorption to things that would otherwise not stand a chance...a boatload of herbals - a common example would be testosterone products that include in particular...curcumin and/or ashwagandha.
And you absolutely can be absorbing things you would otherwise not want to...maybe the best example are with certain fat-soluble vitamins which could prove toxic over time. But, I am hard pressed to think it would happen with respectable use (say a pro-test product you cycle say for 3 months and then take a break from). More concerning that fat-soluble vitamins would probably be some micro-minerals, BUT I have yet to see a reported case of toxicity and this is one of the compounds that actually does have quite a bit of research behind it (mostly centering on about 5 or 6 ingredients; I think Sabinsa sometimes prematurely exaggerates how many items would truly be proven more bioavailable).
Overall, your guess may be as good as mine because it just hasn't even been studied to the level you are requesting (despite having more research than most supplement ingredients) and probably because people just don't take it long enough to be too concerning...at least I would anticipate.
Interesting and fair concern though!