Fountain pens are available with different types of nibs. The most common would be the standard nibs that produces lines with uniform thickness. Next is probably the flex nib which can produce thin and thick lines depending on how much pressure you apply. Lastly, there are specialty nibs such as the stub, music, italic, fude and the Sailor Specialty Nibs which are in their own category.
Flex nibs can be used for drawing, writing and calligraphy. While I've owned and reviewed many flex nib fountain pens, they aren't my favourite pens for drawing with.
Drawing with flex nibs requires more thought. The thickness of lines has meaning in art. A thin line could be used to suggest an element in the background vs a thickness for an element in the foreground. With architecture-style drawings, thick lines are used to outline the outer shapes or exterior form while thin lines are used to draw details or elements that do not contribute to physical change of form. Cross hatching can only be created with thin lines.
Whether you enjoy drawing with flex nibs really comes down to personal preference. And sometimes you won't know whether you'll like flex nibs until you've tried one.
In the fountain pen world, there are categories for flex nibs, such as soft vs flex, vintage flex vs modern flex. This blog post from Goulet Pens actually talks more about the differences.
Generally speaking, any pen nib that can produce line variation depending on the pressure applied can be called a flex nib. Vintage flex nibs are old nibs which are said to be softer and more flexible compared to modern flex nibs which are mass produced. I would describe a soft nib as one that feels like you're drawing with a brush springy bristles.
Pilot Namiki Falcon with Spencerian modification
The only pen I have that with a "soft" nib is the Pilot Namiki Falcon with Spencerian modification. The standard Namiki Falcon nib is just a normal flex nib.
A standard Pilot Namiki Falcon fountain pen is around US $170. I got mine modified by Nibs.com for an extra US $140. Is it worth it? Most definitely yes if you're looking for a soft flex nib.
The Spencerian modification makes the Falcon nib thinner/sharper and slightly softer. The additional softness is enough to make the pen feel very different compared to all other flex nibs I've ever used. And it's a very noticeable difference. And because the tip is so sharp, you can get fantastic line variation.
BlueDew fountain pen
The BlueDew fountain pen uses stainless steel flex nib. In terms of performance, it's similar to the Zebra G nib that's popular for drawing comics.
This is a great pen for people who want the performance of a Zebra G nib with the convenience of having an ink converter. The pen is priced at US $88 and is sold by a company based in Singapore.
Osprey fountain pens that use Zebra G nib
Osprey Pens makes fountain pens that can be fitted with the Zebra G nib. The cheapest pen that can be used with the Zebra G nib is the Osprey Madison which is just US $30.
The difference between Osprey Pens and BlueDew is the actual Zebra G nib can get rusty with time whereas BlueDew's stainless steel nib will not rust.
You can check out my reviews for Osprey Pens at https://www.parkablogs.com/tags/osprey-pens
The other Pilot Falcon nib
This is the other Pilot Falcon nib that's found on the Pilot Custom 743 and the Pilot Custom 912. The nib is slightly stiffer and thicker compared to the Pilot Namiki Falcon nib.
These are probably the better modern flex nibs I've used.
Other flex nibs
Here's a list of other flex nib fountain pens with rather similar performance:
The more interesting ones would be the Noodlers Neponset and Noodlers Triple Tail which can produce thicker and very wet lines.