There are two things at issue here -- design and manufacture.

The design of the OHV motors is pretty good. The big problem
with these cars is the initial materials used when they were
built originally. This shouldn't be a surprise, since the B230
has another 20 years of metallurgy to contribute to chosing the
proper materials when the thing was made, and the cars also have
only ever been exposed to modern lubricants while the B20s had
to deal with a decade or two of oil that was far worse than even
generic dino oil today.

The issues with the B18/B20 are:

weak cams / lifters (maybe nobody knows which goes first),
which seem to be a design defect but one that is easy to address
either by using good oil or by replacing the cam and lifters.
Once this is done once, there doesn't seem to be a need to do it
again. It is likely that the majority of cars out there have
already had this done.

Weak rings: many of these cars seem to break their rings at
some point. This may be because it is possible to overheat one
of these cars and not have anything obviously bad happen like on
a motor with an aluminum head which will warp. The one bad
thing will be that the tempering on the rings will be ruined and
the rings will break. Even with broken rings, these motors run
okay but have bad blowby.

The valve seats tend to wear out, but this is because the
motors were designed to run on leaded fuel and at high rpms
the valve seats may erode over time. Again, the B21 and up are
built with hardened valve seats (and must have these because the
head is aluminum and this *requires* hardened seats). I'm not
sure if I would call this a design flaw since the B18/B20 were
designed to run on leaded gas.

Lastly, these motors require timing gear changes. This is
not a big deal, since nothing bad happens if the gear breaks,
the gear tends to break gradually and give plenty of warning
before the car doesn't move, and the gear can be replaced with
either a steel unit or an aluminum unit which will likely give
a lifetime of service.

The B18/B20/B30 are better than the B21 and up because they
don't need new timing belts, they can have their valves adjusted
without gettings shims, and because if you overheat one you
don't need to have the head skimmed.

The B18 turned into the B20 by expanding the bore. The B20
turned into the B30 by adding 2 more pistons. The B20, in 1974,
had it's innards changed to have metric sized bearings and it
got an 8 bolt connector for the flywheel. The B21 uses this
exact same crank and the same connecting rods as the metric B20,
and even the pistons are the same on the lower displacement
ohc motors used in some markets. The clutch, bearings, crankshaft
connecting rods, and probably a few other things are the same
between the late B20 and the B21.

I'd say that the B20 is a better motor than a B21/B23/B230,
if the B20 has been rebuilt once and had the cam, valve seats
and timing gear updated. This may or may not mean that the B21
is a better motor than the stock B20.
chris