One drawback of using Mojo::Base and Object::Simple is its similar but slightly different and incompatible syntax with the Moo* family, so your code is not "upgradable" to Moo or Moose once you need more features. And often you'll end up wanting them, e.g. one day you'll probably read about the wonders of method modifiers (before, after, around), or roles, or wanting to have a lazy constructor, or triggers, and so on.
I'd recommend instead Mo. It's more lightweight than Object::Simple and you can do default value, builder, ro/rw, required, even coercion. But the features are modular and you only pay for what you use. And once you need more features later, you normally should be able to just replace 'use Mo' in your code with 'use Moo' or 'use Moose'.
Of course, this point is moot if you don't care about compatibility/upgradability to Moo*.
I am glad someone took Mojo::Base and made a module out of it. I've tested several minimal pure-Perl class builders and this one came out really fast. Only Object::Tiny is a tad faster, but the difference is irrelevant. In terms of features it does the basics so you don't have to write your accessors by hand. Doesn't bother with constraints ro/rw accesors, etc. If you need that, use Mo or something else. This module does not deserve a bad review.
I don't always oppose reinventing the wheel, in the case of this module, however, I wonder if it is worthy to be registered at any cost on CPAN.
You already have Moose, Mouse, etc. and otherwise Class::Accessor if you dislike the previous modules. Enough already! Do you need yet another but half-baked module, nonetheless?
I hear the author says that he is going to enlighten the perl's newbies, to be honest, however, in no case is he qualified to do so. In fact, he seems to be conceited because of the publication of his only book regarding Perl.
In any case, I fear that what he says and does would mislead the newbies (especially Japanese) deceived by another handle name of his, "perlcodesample".