Drawbacks to modular synthesis

Despite being extremely versatile there are drawbacks to modular synthesizer systems.

What do you aim to achieve with a modular synth?
Back in the day, modular was the only way synthesizers existed, when other synths came along the people who wanted to play music moved on. Modular excels at generative experimental stuff especially for those that don’t want to use software for that purpose. A modular synth (especially Eurorack) can be regarded as an audio signal processing and routing system. Whether modular is for you depends on what kind of music you want to make (if any). Conventional synths are optimized to produce conventional music. Keyboard based polysynths are very good at playing piano and organ parts. Modular synths can only do that with great effort and expense. It is difficult to play chords on a monosynth except as arpeggios. If you want polyphony, modular is not ideal.

Modulars without patch memory have no preset sounds. A patch can take time to get right. One patch can't be switched quickly to another. It can be difficult to reproduce a patch. The dangle of cables obscure the controls and you can spend time reorganising modules just to make cable management easier.

Modular allows you to completely customize your instrument and it's very tactile. If you plan well each module added to your system can add significantly more functionality. It can be used for generative experimental stuff, or as a weird noise maker or it can be played as a monosynth from a keyboard or sequencer. A modular can do a bit of everything (drums, bass, lead and so on) but only within specific sets of sounds based on what modules you have.

Modular is probably the least cost-effective way to make music. For one synth voice, need at least an oscillator, filter, VCA, modulation (envelopes and/or LFOs, preferably both). You'll need something to trigger the sound and to tell it what pitch to play, and utility modules, a power supply and case to house it all. If you're mostly using it for standard subtractive synthesis modular is way too expensive. A keyboard synth, semimodular or desktop synth at a quarter or less of the price will do far more than a monosynth. In the early stages of building and using a modular system, you'll run into limits and realize you need more modules or different modules or that you need to rethink your approach. GAS (gear acquisition syndrome): your system will always be open to improvement.