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Using SMR (Shingled Magnetic Recording) HDDs in a NAS can have drawbacks depending on your use case. Here’s a breakdown of whether it's "bad" and the key considerations:

What Are SMR Drives?
SMR drives overlap data tracks on the disk platter, similar to shingles on a roof, to increase storage density. This design can lead to slower write speeds and other performance issues because rewriting overlapped tracks often requires rewriting adjacent tracks.

When It's Okay to Use SMR Drives in a NAS
1. Archival Storage
   If your NAS is primarily for cold or archival storage (i.e., write-once, read-often workloads), SMR drives can work well due to their large capacities and lower costs.
   - Example: Storing backups, photos, videos, or rarely-changing data.

2. Low Write Workloads
   For environments with infrequent writes or where write speeds aren't critical, SMR drives might perform adequately.

3. Budget Constraints
   SMR drives are often cheaper than CMR (Conventional Magnetic Recording) drives, so they may be a cost-effective choice for light NAS use cases.

When SMR Drives Are a Bad Choice
1. RAID Configurations
   SMR drives can cause problems in RAID arrays due to their slow write speeds and potential for high latency when rebuilding a degraded array. This can lead to:
   - Longer rebuild times, increasing the risk of another drive failure.
   - Reduced overall performance for the array.

2. Frequent Writes or Heavy Workloads
   If your NAS handles:
   - Frequent file updates or deletions.
   - Data-intensive tasks like video editing or database operations.
   - Continuous writes (e.g., surveillance footage).
   Then SMR drives may struggle due to their slower random write speeds.

3. Applications That Need Consistent Performance
   SMR drives can experience performance degradation during sustained writes or when their internal cache (a small CMR section) fills up. This can cause:
   - Noticeable delays in file transfers.
   - Inconsistent response times.

4. Shared Environments
   In multi-user NAS setups, SMR drives may bottleneck performance when multiple users access or write data simultaneously.

How to Tell if an HDD Is SMR or CMR
Manufacturers don’t always clearly label SMR drives. To identify them:
1. Check the product datasheet or specifications online.
2. Look for terms like "shingled magnetic recording" or "SMR."
3. Research user reports or reviews for confirmation.

Best Practices
- Use CMR Drives for Critical Workloads: CMR drives, also known as PMR (Perpendicular Magnetic Recording) drives, provide better performance and reliability for NAS use.
- Mix Drives with Care: Avoid mixing SMR and CMR drives in the same RAID array to prevent performance mismatches.
- Consider the Manufacturer’s Recommendations: Some NAS manufacturers explicitly recommend or certify drives for use in their systems. Check the compatibility lists (e.g., Synology, QNAP).
- Monitor Performance: If you use SMR drives, monitor your NAS for performance issues, particularly during RAID rebuilds or heavy write operations.

Conclusion
SMR drives can be used in a NAS for light, archival workloads, but they are a poor choice for write-heavy or performance-critical applications, especially in RAID arrays. For most NAS setups, CMR drives like those in Western Digital’s Red Plus or Seagate’s IronWolf lines are a safer and more reliable option.