There are different opinions and views within virtue ethics as to which virtues to follow and, more importantly, what makes a "Virtuous person" the contrast in opinions demonstrates the relativism of Virtue ethics.
The doctrine of the mean does not work in practice
It seems perfect in theory but as the statement claims, it is difficult to work in practise. The main area of failure is when/ how we know when we've gone to either extreme? This judgement amongst subjective beings will never have an objective answer creating many problems.
Can you have an excess of love or compassion?
Each virtue has a vice in virtue ethics. It can be seen that an excess in love or compassion could result in injustice as we may treat someone in a different manner due to excess love when they are not worthy of it. An example of a court room may be given.
Acting moderately is the best way to act towards the environment
The term 'moderately' would mean different things within different contexts. In an environmental context it wouldn't necessarily mean that sometimes it is ok to litter for example. Rather it would mean that a balance is kept with the amount of time and effort that is put towards the environment if we go to an extreme with this (time and effort) then other more important aspects in our lives (e.g family/work) may become weakened.
Agent-centred virtues are inherently selfish
Performing an act of good in order to receive anything other than ultimate success (which to me is doing it for God alone) has lost its value due to the intention.
If Hitler was chosen as a role model would virtue ethics still work?
Some people do see Hitler as a role model but following in his path would logically go against the very definition of virtue and therefore virtue ethics would not work.
Who are we to tell other cultures what virtues they should aspire to?
Culture itself should be separated from the aspirations of virtues as we as human beings are not