it starts out at 20% and you usually don't get it first and tend to max it last. It's a good bit of sustain but it is not even half of what you are talking about unless you are maxing it first for some reason. The Real value on this is the active not the passive since it can give you a second health bar and let you finish off whoever or run etc.
Also it Should be noted that his devour isn't the first %health damage nuke. Garen and Jinx have similar executes in their kits. Furthermore his AA is weak especially pre 6. His Q does moderate damage but at max range will only let you just catch up (0 slows on yourself) before they are racing off again. Even Malzahar can kite you.
His best point in the game is mid game IMO. Early game he is ok but you can't bring much damage and your utility is average for what you need. Late game everyone outscales you except for maybe other tanks. You have to pick who you want to counter/role as bruisers will just sustain off you, adcs will kite+sustain off you, and mages will just poke/kite you and you don't have the damage to run in and kill them before your cc wears off and they run from you.
Finally I don't know why people keep building a TF when the multiplier isn't affected by your ult passive. It comes out as 2-3 sources of damage. IMO you should have swiftness boots, roa, frozen mallet, spirit visage, and randuins as your standard build with the last item being situation. IF you are behind though drop roa for a frozen heart.