A restriction would only be of any value if coupled with significantly increased Driver Training. There needs to be mandatory Advanced Driver Training, including motorway driving before the full licence can be awarded. This would do more for road safety than a restriction alone, considering the majority of new drivers will be driving a car that falls within any proposed power restriction anyway. A Vauxhall Corsa driven badly is still a potentially fatal hazard.
As far as Motorcycles are concerned; the under 19 125cc restriction is wise.
I could argue for a longer, more in depth CBT course to give learners a more solid foundation to build their experience onto, however, a learner must do just that, make mistakes, analyse and improve. There is only so much that can be done to smooth out that process.
For those aged 21 and over the time restriction upon passing their Standard Motorcycle Test is extreme, illogical and unnecessary. 2 years is a long time, and in no way guarantees that the individual will be any more prepared to ride a larger displacement bike when the restriction ends. 6-12 months with mandatory Advanced Rider Training on a bike with a minimum of 35kw power output would be a far superior route for rider safety and competence.
This 2 year restriction causes many individuals to prematurely remove their restriction kits on more powerful machines; effectively riding uninsured and with no licence, having no prior training on a larger bike, which is a danger to everyone. 6-12 months restricted with mandatory Advanced Rider Training would ultimately form better, safer motorcyclists.
The Advanced Rider Training after completing the test should also be applicable to those who have passed via Direct Access.
While restrictions can have some value, when logical, in the case of the under 19 125cc limit for example.
Ultimately: Increased Training > Increased Restrictions.