Oliver Heaviside and Lord Kelvin both considered it as an evil concept and strongly believed that Maxwell was misled by using it to derive Maxwell's Equations. Heaviside then come up with the famous modern version of Maxwell's Equations with four equations but now ironically all of them can be represented with one concise single equation using quaternion based on geometric algebra [1]. It's probably worth to mention that Einstein was using quaternion concept when discovering his infamous special relativity [2].

It's interesting to note that complex number was once had the same fate as quaternion and it was also considered as an evil concept at the time by many well-known mathematicians not unlike the concept of zero/sifar/cipher when it's first invented and then popularized by the Arab mathematicians. Modern engineering achievements for example the wonderful world of mobile and digital communication are all based on complex number concept namely in-phase and quadrature or I/Q for short. This venerable concept of I/Q however is limited once you wan to include and consider polarization of the EM wave and this is when quaternion become very handy and useful [3]. I'd not be surprised that the next generation of wireless mobile i.e. 6G will be based on this new 3D quaternion based I/Q or I/Q/P (not yet a concept maybe soon), similar to the impact of I/Q as the basis of successful 2G to the 5G with their OFDM.

[1] A derivation of the quaternion Maxwell’s equations using geometric algebra:

http://peeterjoot.com/2018/03/05/a-derivation-of-the-quatern...

[2] Quaternions in University-Level Physics Considering Special Relativity:

https://arxiv.org/ftp/physics/papers/0308/0308017.pdf

[3] Modeling dual polarization wireless fading channels using quaternions:

https://ieeexplore.ieee.org/document/1708024