At a previous $dayjob at a Tier 1, we would only support LAG for a customer L2/3 service if the ports were on the same card. The response we gave if customers pushed back was "we don't consider LAG a form of circuit protection, so we're not going to consider physical resiliency in the design", which was true, because we didn't, but it was beside the point. The real reason was that getting our switching/routing platform to actually run traffic symmetrically across a LAG, which most end users considered expected behavior in a LAG, required a reconfiguration of the default hash, which effectively meant that [switching/routing vendor]'s TAC wouldn't help when something invariably went wrong. So it wasn't that it wouldn't work (my recollection at least is that everything ran fine in lab environments) but we didn't trust the hardware vendor support.