You are carrying the extra weight (sealant plus tube)
Filled tubes don't actually seal as well as just tubeless (the tube moves against the tyre and keeps breaking the seal, it's more noticeable with larger holes).
You can't run the low pressures that you can with tubeless, as you may get pinch damage to the tube (leading to large holes in the tube, which might not seal), low pressures (and its knock on effects of lower rolling resistance and more traction) is the single biggest advantage of tubeless. FWIW I run between 20 and 30 psi on all my MTBs, so does the wife. Her 29er and our CX bikes (all running tubeless) have been down below 20.
Eventually the tube will stick to the inside of the tyre due to the sealed punctures (thorns etc), and you *might* end up splitting or tearing the tube.
Many tubes are filled with powder (chalk or talc), you can end up with a solid lump of rubber bouncing round your tube, mixture of the powder and sealant. (You get this with proper tubeless as well, but it takes ages. >12 months usually.)
Can't use tubes with latex in them (not with latex based sealants anyway).
If you are just doing it to puncture proof a hack/winter/commuter bike, it's good. For anything else, you may as well go tubeless.
BTW, if you have some old tubes, you *could* go ghetto tubeless. Which won't cost you anything but the sealant and/or a couple of 24" tubes.