1. A level 10 Lugia needs an exp. share because it was only level 10. You obviously don't send a level 10 Pokemon out to battle in the wild unless against Pokemon of similar level.
2. You probably sent Lugia out to face something it was weak against. Since it doesn't have Multiscale before Generation V, it can't take hits as well. However, Lugia has unbelievable defences, so you probably sent it out under levelled as well, or it just had incredibly bad IVs (even so, it'll still have had really high defences). Lugia is also not an attacking Pokemon, so if you were using it for that, you weren't letting it play to its strength. Lugia's there to toxic stall and/or take hits.
3. Deoxys is not overrated. All of its forms serve very specific purposes and it does its specific jobs very well. Its speed form is there to set up quickly and Taunt any other set up Pokemon. Its attack form is there to sweep. Its defence form is there to wall.
4. Pikachu is **not** overrated. It's frail and useless without a Light Ball, even EV trained. No one uses Pikachu unless in it's in NU with a Light Ball or if they're new to battling and just want to recreate Ash's team.
5. As Starfire said, Metagross and Bronzong serve two very different purposes. Metagross is there to hit hard, and Bronzong is there to take hits & set up. Neither are overrated, considering their advantageous typings, their useful abilities, their few weaknesses and their numerous resistances.