One good turn deserves another; so I'd like to add onto what
pika pal said since he done it for me before.
The problem is that theory almost always turns out to be a lot different to what actually happens in practise. Take (non-custap) Forretress, for example - it seems incredibly solid were you just to look at what it has at face value. It has access to almost all types of hazards aside from Webs, very solid defences, only one weakness, the ability to rapid spin, a guaranteed turn of set-up (fake out and the like notwithstanding) with Sturdy, and even the ability to pivot with Volt switch. So at face value you might assume Forretress to be one of the better leads or defensive Pokémon in the game; but in practise it finds itself just sort of sitting there and not doing anything. It has no offensive presence, it leads to free turns for your opponent, has no sustainability aside from leftovers and there's honestly a ton better hazard removers in the game; especially with the rise of Defog.
A lot of the same can be said for Metagross. On paper it might seem solid with all the good points you've found, but in practise it's really underwhelming and before the discovery of AV Pursuit was the reason it wasn't even on the viability rankings and went to UU for a long time before ORAS gave us Metagrossite. It provides almost nothing to a good team, it has too many weaknesses to effectively be used purely defensively, offensively it has far too much competition to be taken seriously, it has no way to boost it's attack aside from things like Power-up Punch and has no good ability for this role... realistically, unless your team really needs a Pursuit Trapper with Jirachi's defensive typing, when using it you'll be really underwhelmed and wishing you picked something else.
At the same time, if you want to use it and feel you can find a new unique use for it, go straight ahead. It's always nice to see something good and new rise in OU; I just highly doubt you'll find it in base Metagross.