Stability and performance are the primary goals for the FreeBSD base system.
FreeBSD seems to often be the target of false criticism from people who have little or no experience with it. If someone tells you that FreeBSD is "way behind", "not up to snuff", etc., take the Socratic approach: Ask them to describe some of its shortcomings in detail and watch them demonstrate their lack of knowledge.
In reality, FreeBSD is a very powerful, enterprise-class operating system, used in some of the most demanding environments on the planet. A short list of FreeBSD-based products and services you may be familiar with is below. See https://en.wikipedia.org/wiki/List_of_products_based_on_FreeBSD for a more complete list.
You may hear that FreeBSD is not as cutting-edge as some of the Linux distributions popular for personal use. This may be true from certain esoteric perspectives, but the reality is that only a tiny fraction of programs require cutting-edge features that FreeBSD lacks and FreeBSD is capable of running virtually all the same programs as any Linux distribution, with little or no modification.
A reliable platform on which to run them is far more important in scientific computing and there is no general-use operating system more reliable than FreeBSD.
Enterprise Linux offers comparable reliability, but FreeBSD offers newer compilers and libraries than Enterprise Linux, making it easier to build and run the latest open source software.
The FreeBSD ports collection offers one of the largest available collections of cutting-edge software packages that can be installed in seconds with one simple command. Users can also choose between using the latest packages or packages from a quarterly snapshot of the collection for the sake of stability in their add-on packages as well as the operating system itself. The quarterly snapshot's receive bug fixes, but not upgrades, much like Enterprise Linux distribution.
FreeBSD ports can be easily converted to pkgsrc packages for deployment on Enterprise Linux and other Unix-compatible systems.
FreeBSD has a Linux compatibility system based on CentOS. It can run most closed-source software built on RHEL/CentOS, although complex packages (e.g. Matlab) may be tricky to install. Ultimately, though, FreeBSD is actually more binary-compatible with RHEL than most Linux distributions. It uses tools and libraries straight from the CentOS Yum repository. The RPMs there are easily converted to FreeBSD ports for quick, clean deployment on FreeBSD systems.
Note that the compatibility system is not an emulation layer. There is no performance penalty for running Linux binaries on a FreeBSD system, and in fact some Linux executables may run faster on FreeBSD than they do on Linux. The system consists of a kernel module to support system calls that exist only in Linux, and the necessary run time tools and libraries to support Linux executables. The system only requires a small amount of additional RAM for the kernel module and disk space for Linux tools and libraries.
Hence, if you are running mostly open source and one or two closed-source Linux applications, FreeBSD may be a good platform for you. If you are running primarily complex closed-source Linux applications (Matlab, ANSYS, Abaqus, etc.), you will likely be better off running an Enterprise Linux system.
ZFS is fully-integrated into the FreeBSD kernel, and is becoming the primary file system for FreeBSD. The FreeBSD installer makes it easy to configure and boot from a ZFS RAID.
The UFS2 file system is still fully supported, and a good choice for those who don't want the high memory requirements of ZFS. UFS2 has many advanced features, such as an 8 ZiB file system capacity, soft updates (which ensure file system consistency without the use of a journal), an optional journal for quicker crash recovery, and backgrounded file system checks (which allow the file system to be checked and repaired while in-use, eliminating boot delays even if the journal cannot resolve consistency issues).
There are many other advanced features and tools such as FreeBSD jails (a highly developed container system), bhyve, qemu, VirtualBox, and Xen for virtualization, multiple firewall implementations, network virtualization, and mfiutil for managing LSI MegaRAID controllers, to name a few.
FreeBSD is a great platform for scientific computing in its own right, especially for running the latest open source software. It's also a great sandbox environment for testing software that may later be run on RHEL/CentOS via pkgsrc.