lanesharing

Is PCIE lane sharing that bad these days esp with PCIE Gen 5? As even a RTX 5090 does not fully saturate a PCIE 5 x8 slot, what is the big deal if the gpu runs at PCIE 5 x8 or PCIE 4 x8 (eg. in case of 4000 series cards) speeds? esp. mid tier GPUs? Have any of you had problems with lane sharing...