I have had relatively good results from Hotels.com. e.g. last holiday in Canada we stayed a few days in the same hotel at the beginning and end of an RV holiday. At the start (4 nights) we got a very nice suite upgrade. At the end (1 night) we got the best suite in the whole hotel! And that was a booking using some of our free nights credit to reduce the cost. Sadly Hotels.com has changed its terms, so won’t be using them anymore.

Bookings.com, never ever had an upgrade. Likewise the major chains, rarely have an upgrade (though to be fair I only have ‘medium’ status in the two I use regularly).

YMMV.