Alcohol protection with ethers Various ethers are used for hydroxyl protection. My question is about their removal. I know that there are various methods but I want to specify on the use of acids.

Certain groups are removed easily like Trityl, by using aqueous acetic acid. If I'm not mistaken, the mechanism is Sn1, as the trityl carbocation is stabilised through delocalisation on the phenyl groups.

However, other similar groups like benzene or PMB are not removed that easily. Can't they follow the same logic, as the postive charge could again be delocalised in the aromatic ring? Is it because the delocalisation is not good enough, as in the case of trityl?

Also, I've noticed that in order to remove smaller groups like methyl, haloacids are mainly used. Why can't a more weak acid do the job?
You are right.In case of benzene and PMB, the delocalization is not good enough. So a strong acid is required.

As for the case of OMe, a strong acid as well as strong nucleophile will be needed. A weaker acid can't do the job. Let me explain why.
Note that carbon oxygen bond is quite strong. 
So what we need is a strong nucleophile. But in case of OMe as a protecting group, even a stronger nucleophile will not work if the oxygen is not heavily protonated due to two reasons:-
1. Strength of C-O bond.
2. SN1 can't occur as methyl carbocation not stable.

Here acid is a stoichiometric reagent, not simply a catalyst. If a weaker acid is used, it will just establish a protonation-deprotonation equilibrium. However, a stronger acid will shift the equilibrium towards protonation.
And haloacids are best suited.

Conclusion being that a weaker acid will render nucleophile incapable to cleave ether in case of OMe.